WASHINGTON — People signing up for health insurance through the Affordable Care Act’s federal and state marketplaces tend to be older and potentially less healthy, officials said Monday, a demographic mix that could threaten the law’s economic underpinnings and cause premiums to rise in the future if the pattern persists. 
 
 Questions about the law’s financial viability are likely to become the next line of attack from its critics, as lawmakers gear up for the midterm elections this fall. Republicans quickly seized on the government’s progress report on Monday as evidence that the health insurance law would not work. 
 
 But administration officials expressed optimism that more young people would sign up in the months ahead, calling the latest enrollment numbers “solid, solid news” for the health care law. They said that interest in obtaining insurance through the marketplaces was increasing sharply across all age groups and that youth outreach efforts would become more aggressive as the March 31 open enrollment deadline approached. 
 
 “We’re pleased to see such a strong response and heavy demand,” said Kathleen Sebelius, the secretary of health and human services. “Among young adults, the momentum was particularly strong.” 
 
 Of those who signed up in the first three months, administration officials said, 55 percent are age 45 to 64. Only 24 percent of those choosing a health insurance plan are 18 to 34, a group that is usually healthier and needs fewer costly medical services. People 55 to 64 — the range just below the age at which people qualify for Medicare — represented the largest group, at 33 percent. 
 
 The latest figures about enrollment add pressure on the Obama administration after a disastrous rollout of the HealthCare.gov website in October. Senior officials said they understood the stakes and were working to increase sign-ups. The White House recently hired Marlon Marshall, the deputy national field director for Mr. Obama’s 2012 presidential campaign, to run a campaign-style effort aimed at increasing sign-ups, especially among young people. 
 
 Brendan Buck, a spokesman for the House speaker, John A. Boehner, Republican of Ohio, predicted that the White House would fail to meet its goals and said that insurance premiums would rise. 
 
 “There’s no way to spin it: youth enrollment has been a bust so far,” Mr. Buck said. “When they see that Obamacare offers high costs for limited access to doctors — if the enrollment goes through at all — it’s no surprise that young people aren’t rushing to sign up.” 
 
 The demographic information, which had not been broadly available until Monday, also offers the first concrete evidence about whether the national health care experiment will work the way it has in Massachusetts, where a government marketplace also offers insurance to people who do not receive it through their employers. Officials said they were optimistic because the pattern of sign-ups among young people looked similar to the one they had seen in that state, which had a surge in sign-ups as the deadline approached. 
 
 Over all, officials said that 2.2 million people had signed up by Dec. 28 for health insurance through HealthCare.gov and the state-based websites. Administration officials have previously said they hope to see seven million people enrolled in private health plans through the federal and state exchanges by March 31. 
 
 Of those who signed up, about 54 percent are female and 46 percent are male. Nearly 80 percent of those who selected a plan qualified for federal subsidies to reduce their premiums, officials said. Federal officials said they did not know how many people selecting plans were previously uninsured or how many have paid premiums. People are required to pay their share before coverage takes effect. Officials also did not know the race or ethnic origin of those who signed up. 
 
 The age breakdown was the most highly anticipated data being released because of what it could say about the health of those who will be insured. Under the law, insurers can no longer deny coverage or charge higher premiums because of a person’s medical history or pre-existing conditions. As a result, White House and health policy experts have repeatedly said that insurers need to sign up large numbers of younger people to balance the financial risks of covering older Americans who require more medical care. 
 
 Larry Levitt, a senior vice president of the Kaiser Family Foundation, has said that “the mix of enrollment is much more important than the total number.” 
 
 “If you assume that sicker individuals are likely to come in first, then a smaller pool is likely to be a sicker pool,” Mr. Levitt said. “The best guarantee of a diverse pool is a big pool, because that means you are probably reaching younger and healthier people.” 
 
 Mr. Levitt said that people enrolling early included some with the greatest needs: people who had been locked out of the individual insurance market because of serious illnesses and those coming from federal and state programs for people with pre-existing conditions. 
 
 The report from the administration showed that older Americans accounted for a large share of those choosing health plans in a handful of states, including West Virginia and Wisconsin, where 66 percent were age 45 to 64. In Maine, 64 percent were in that age bracket, and in Arkansas their share was 63 percent. 
 
 Robert Laszewski, a consultant who works closely with insurers, said, “You need healthy people of all ages,” and so far, he said, “the program is not ramping up fast enough to guarantee a good balance of healthy and sick people, which you need to sustain the program.” 
 
 Robert E. Zirkelbach, a spokesman for America’s Health Insurance Plans, a trade group, said “it’s too early to know” if the enrollment of younger and healthier people was adequate to keep premiums stable in coming years. Insurers will need to see enrollment numbers for the full six-month period, he said. 
 
 Many people who bought insurance on their own have received notices saying their policies were being canceled or discontinued because the policies did not comply with coverage requirements of the new health care law. Those expecting high medical bills had the strongest motivation to overcome the obstacles to buying insurance on the federal exchange. 
 
 Caroline F. Pearson, a vice president of Avalere Health, a research and consulting company, said that “early enrollment is skewed toward older individuals.” She added: “Sicker, older folks tend to sign up first because they are more motivated. They are likely to have health care costs early in the year.” 
 
 Anne Filipic, the president of Enroll America, a nonprofit group trying to expand coverage, said, “We know from our research that when uninsured consumers learn about the financial help that is available to them, they are eager to enroll.” 
 
 Health plans in the marketplace are separated into four categories — bronze, silver, gold and platinum — indicating the generosity of coverage, or the share of costs paid by insurance for an average enrollee. Of people choosing plans so far, 60 percent selected silver plans and 20 percent signed up for bronze plans. Thirteen percent chose gold plans, and 7 percent platinum coverage. 
 
 In December, for the first time since the marketplaces opened, the number of people selecting plans in the federal exchange exceeded the number signing up through state exchanges. 
 
 For the first half of the enrollment period, through December, the federal exchange, which serves 36 states with about two-thirds of the nation’s population, accounted for 56 percent of those selecting plans. The other 44 percent were in state exchanges. ||||| WASHINGTON — Nearly 3.3 million people have signed up for health insurance through the marketplaces established by President Obama’s health care law, and about one-fourth of them are young adults, the administration said Wednesday. 
 
 The administration reported a modest uptick in the enrollment of young adults, a group avidly sought by insurers because they are usually healthier and need fewer costly medical services. 
 
 In a new report on enrollment, the administration said that 1.9 million people had selected health plans in the federal marketplace from October through January, while 1.4 million chose plans in state-run insurance exchanges. 
 
 In January alone, officials said, more than 1.1 million people signed up for insurance in the federal and state exchanges. 
 
 Administration officials said they were pleased with the numbers. “These encouraging trends show that more Americans are enrolling every day, and finding quality, affordable coverage in the marketplace,” said Kathleen Sebelius, the secretary of health and human services. 
 
 “The covered population is getting younger,” Ms. Sebelius said. In January, 318,000 people age 18 to 34 selected health plans, bringing the total in this age group to 807,500, officials said. 
 
 The administration’s goal was to have 4.4 million people signed up by now, according to a memorandum prepared in September by the Department of Health and Human Services. But the federal insurance website, HealthCare.gov, got off to a rocky start, thwarting many people who tried to sign up in October and November. 
 
 The new data show that people buying insurance on the exchanges still tend to be older and potentially less healthy. 
 
 Of those who signed up in the last four months, administration officials said, 53 percent are age 45 to 64 -- down slightly from 55 percent in the first three months. About 25 percent of those choosing a health insurance plan are 18 to 34. This group accounted for 24 percent of those picking plans in the first thee months. 
 
 People 55 to 64 -- the range just below the age at which people qualify for Medicare -- represented the largest group, at 31 percent, down from 33 percent in the months from October through December. 
 
 The open enrollment period continues until March 31, and White House officials predict a surge of applications just before the deadline. 
 
 People who go without insurance after that may be subject to tax penalties, although the Internal Revenue Service has indicated that it prefers public education over aggressive enforcement in the first year of the “individual mandate.” 
 
 After reviewing the new report, Caroline F. Pearson, a vice president of Avalere Health, a research and consulting company, said: "Enrollment in the exchanges appears to be on track to reach roughly six million by the end of March. It is also important that the age mix of enrollees improved in January, relative to the first three months of open enrollment. That supports my hypothesis that the older, sicker enrollees will sign up first, and the younger, healthier people will enroll later.” 
 
 After reviewing the new report, Caroline F. Pearson, a vice president of Avalere Health, a research and consulting company, said: “Enrollment in the exchanges appears to be on track to reach roughly six million by the end of March. It is also important that the age mix of enrollees improved in January, relative to the first three months of open enrollment. That supports my hypothesis that the older, sicker enrollees will sign up first, and the younger, healthier people will enroll later.” 
 
 The administration and its allies are planning a big push to sign up more people, in the hope that total enrollment through the exchanges could reach the administration’s original goal of seven million by the end of March. 
 
 About four-fifths of those choosing health plans to date qualified for financial assistance to help pay their premiums, administration officials said. 
 
 Senator Mitch McConnell of Kentucky, the Republican leader, said Wednesday that many of “the new enrollees in Obamacare exchange plans are actually folks who were already insured” or eligible for Medicaid. 
 
 Nearly four years after it was signed by Mr. Obama, the Affordable Care Act remains a divisive political issue, and administration officials say that Republican attacks on the law have made it more difficult for them to persuade people to enroll. 
 
 Opinion polls show that people who are uninsured disapprove of the health care law at roughly the same rate as people who have insurance. ||||| Welcome to Health Reform Watch, Sarah Kliff’s regular look at how the Affordable Care Act is changing the American health-care system — and being changed by it. You can reach Sarah with questions, comments and suggestions here. Check back every Monday, Wednesday and Friday afternoon for the latest edition, or sign up here to receive it straight from your inbox. Read previous columns here. 
 
 The Obama administration has beaten a monthly health insurance enrollment target for the first time, according to data released Wednesday showing that more than 1.14 million people signed up for health plans in January in the new insurance marketplace. 
 
 
 
 The latest enrollment data from the Obama administration show that 3.3 million people have signed up for private health insurance through federal and state insurance exchanges created under the Affordable Care Act. This figure represents all enrollment from Oct. 1 through Feb. 1. It includes both people who have and have not paid their first month's premium. Of those people, 1,146,100 selected their health insurance plans in January, meaning there was a 53 percent increase enrollment last month alone. 
 
 This makes January the first month that the Obama administration has beaten an enrollment target. Back in September, way before HealthCare.gov's botched launch, the Centers for Medicare and Medicaid Services projected that 1,059,900 people would sign up for private health insurance in January. 
 
 However, the Obama administration still falls short of projected, cumulative enrollment by 1 million people, largely due to the anemic sign-ups in October and November. 
 
 This is also a drop-off from December's enrollment number of 1.8 million sign-ups, which both the Obama administration and health policy experts expected. They predicted a rush of enrollments in December, for people who wanted to get covered at the start of the year. They also foresee another surge of enrollment in March, the last month when people can sign up for coverage through the exchange. 
 
 The new enrollment figures also show a slight increase in the number of young adults signing up for coverage. Young adults between 18 and 35 made up 27 percent of the enrollees in January, compared to 24 percent of those signing up from October through December. 
 
 Taken together, this means that 25 percent of those who have enrolled are the young adults that the Obama administration thinks will be healthier, and thus hold down insurance premiums in the exchanges. This is still below the 40 percent target that the Obama administration had previously set for overall exchange enrollment, although the hope, on their part, is that the 25 percent number continues to inch up in February and March. 
 
 The balance of people selecting coverage is shifting slightly toward the HealthCare.gov site rather than the 14 state-based exchanges, most of which had more successful launches than the federal Web site. 
 
 The biggest takeaway here, probably, is that Obamacare is starting to look more and more like the law that the White House had hoped to launch -- not the one where technical glitches made it nearly impossible for shoppers to get in the door. 
 
 KLIFF NOTES: Top health policy reads from around the Web. 
 
 Small businesses get Obamacare relief -- but aren't sure they'll use it. "Small and midsize businesses stand to benefit the most from the latest delay in the health law's employer insurance requirement. But farm co-owner Laura Pedersen doesn't plan to take advantage of it. The Seneca Castle, N.Y., proprietor of a produce and grain farm last year rearranged her employees' schedules and workloads to keep the farm's full-time staff below 50 workers. Her goal was to avoid having to start providing insurance or pay a penalty in 2015 under the Affordable Care Act." Sarah E. Needleman and Rhonda Colvin in the Wall Street Journal. 
 
 For HealthCare.gov work, CGI employees are still in hot demand. "After denigrating the work of CGI and replacing it as the largest contractor on the federal health care website, the Obama administration is negotiating with the company to extend its work on the project for a few months. And the new prime contractor, Accenture, is trying to recruit and hire CGI employees to work under its supervision." Robert Pear and Ian Austen in the New York Times. 
 
 What did Congress mean when they drafted Obamacare's subsidy language? Let's ask congressional staff! "Two staff-level sources directly involved in drafting the law told TPM that no one intended to block subsidies from being administered through the federal website. Instead, they blame the poorly worded phrase on which conservative have based their case on the unusual legislative process through which Obamacare became law. "I don't even understand how the question is being asked," one source said, "except you could go through the law and find all sorts of issues that probably could have been drafted clearer. I think the opponents of health reform are just scanning for inconsistencies and poorly drafted language and trying to exploit them."" Dylan Scott in Talking Points Memo.
– New enrollment data shows that the total number of people who have signed up for ObamaCare remains below target, but the numbers also provide some rare good news for the White House. Some of the odds and ends: January goal met: Last month, 1.1 million people signed up on either a federal or state exchange, beating the target of 1 million. That's a first, reports the Washington Post. Overall figure shy: The new numbers mean that 3.3 million people have signed up since Oct. 1, shy of the goal of 4.4 million for this point, reports the New York Times. (That goal was set before the disastrous website rollout.) More young people: People ages 18 to 34 accounted for 27% of enrollees in January, up from 24% in December, reports NPR. Overall, that means 25% of enrollees are in this crucial age group, well below the administration's goal of 40%. But health chief Kathleen Sebelius sees hope in the uptick: "The covered population is getting younger." More women: They account for 55% of total enrollees at this point. Big picture? "The biggest takeaway here, probably, is that Obamacare is starting to look more and more like the law that the White House had hoped to launch—not the one where technical glitches made it nearly impossible for shoppers to get in the door," writes Sarah Kliff at the Post.