More Young Adults Have Health Insurance, Study Says Stephen Crowley/The New York Times Copies of the divisive 2010 Affordable Care Act, President Obama's health care overhaul, in the office of Mitch McConnell, the Senate Republican leader, and at the headquarters of Families USA, an advocacy group that supported it. A provision allowing young adults to stay on their parents" policies took effect two years ago. WASHINGTON - The share of young adults without health insurance fell by one-sixth in 2011 from the previous year, the largest annual decline for any age group since the Centers for Disease Control and Prevention began collecting the data in 1997, according to a new report released on Monday. The estimates are drawn from a federal survey of about 35,000 households. It did not ask how the newly insured obtained coverage, but the study's author, Matthew Broaddus, a research analyst at the liberal Center on Budget and Policy Priorities, said the increased coverage for young people was almost certainly due to a provision in the Obama administration's Affordable Care Act that allows children to stay on their parents" insurance policies until their 26th birthday. Joseph Antos, a health care policy expert at the conservative American Enterprise Institute, agreed that the provision of the new law was the only plausible explanation for the increase. He pointed out that young people have been among the hardest hit in the recession and would otherwise have been expected to be less likely to be insured. "Nothing else went well for this age group," he said. The share of people ages 19 to 25 who lacked health insurance fell to 27.9 percent, down from 33.9 percent in 2010, or about 1.6 million fewer uninsured people, according to the data from the federal study, known as the National Health Interview Survey. For the next age group - those 26 to 35 years old - the share of the uninsured rose, a further sign, Mr. Broaddus said, that the health care law was driving the decline among the younger group. The dependents" provision of the health care law took effect in September 2010, and the number of uninsured young adults declined that year, too, though by a smaller amount. Obama administration officials said that drop showed that the law, which was broadly opposed by Republicans, was having an effect. The decrease in the number of young adults without health coverage helped drive an overall decline in the share of uninsured people in the country, the first since 2007, before the recession. The share of all Americans without health insurance stood at 15.1 percent in 2011, or about 46 million people, the center reported, down from 16 percent in 2010. It remained above the level in 2007, when 14.5 percent of Americans lacked health insurance. Before the dependents" provision took effect, young adults were typically forced off their parents" plans at 18 or 21, after high school or college. The C.D.C. data for 2011 captured the first full year that the provision allowed young people to stay on their parents" policies. The report comes two days before the Census Bureau is to release its annual report detailing changes in health insurance coverage for the nation. That report, based on data from the Current Population Survey, is considered the standard because it includes information from about 100,000 households, a very large sample. The C.D.C. data is drawn from a smaller sample, and Mr. Antos said it was likely to indicate the direction of the Census Bureau's report, though that is not always the case. The C.D.C. says its numbers are preliminary and subject to revision. Children under the age of 18 are far more likely to be insured than adults, according to the C.D.C. Just 7 percent of children lacked health insurance in 2011. The poor are among the least likely to have coverage. Forty percent of poor Americans under age 65 lacked health coverage last year - still high, but an improvement over the 42 percent who were uninsured in 2010, according to C.D.C. estimates. The findings are in line with those of another study, released in June by the National Bureau of Economic Research, which found that young adults were a third more likely to be on their parents" employer policies since the provision on dependents went into effect. That study, based on a different data set, the Census Bureau's Survey of Income and Program Participation, which tracks individual households over time, allowed researchers to pinpoint the details of the change in coverage for young adults. Kosali Simon, a professor of health economics at Indiana University, Bloomington, and a co-author of the study, said that only half of that increase was made up of dependents who did not have any coverage to start. The other half, she said, had some form of coverage but switched to their parents" policies.