The most 20 powerful US CEOs 40 and under The most 20 powerful US CEOs 40 and under - U.S. business Sorry Zuckerberg, but don't worry, we're sure you'll make this list next year By Jacquelyn Smith, Mark Zuckerberg is only 27, but the company he's chief executive of, Facebook, may be worth anywhere from $75 billion to $100 billion after it filed for a $5 billion initial public offering earlier this month. Zuckerberg has achieved a great deal since he co-created Facebook in his Harvard dorm room eight years ago - but he doesn't make our list of powerful young CEOs, because his company isn't yet selling shares on the stock market. So, who are the CEOs under 40 running the biggest publicly traded companies, who have to answer to shareholders and carry all the responsibilities of running a business listed on a major exchange? Here are the top 20. Some of them started their own businesses, while others joined established ones and quickly ascended. There are also lucky execs who knew all the right people, and some who took over family businesses. Regardless of how they got there, these young chief executives are the heads of the country's biggest publicly traded companies by market capitalization, as of Feb. 13, that have CEOs 40 and under. Forbes.com: America's 20 most powerful ceos 40 and under They lead a diverse list of businesses, including sports car makers, online real estate sites, property insurers, footwear manufacturers and Internet giants. Larry Page, 38, heads a company whose name has become a colloquial verb. We almost all say "I'll Google it" when we want to research something, as the search engine has become the world's go-to place for information, video, mail, maps and images. Larry Page and his co-founder Sergey Brin were both Stanford University graduate students when they built a search engine they called "BackRub" in 1996. Two years later, the computer science students created Google - a play on the word "googol," which is the mathematical term for a 1 followed by a hundred zeros. The name signifies the enormous volume of information available on the site. Today Google has a market cap of $199.05 billion. Andrew Mason, 31, is the founder and chief executive of Groupon, the popular "daily deals" website that offers users deep discounts on local businesses. Mason graduated from Northwestern University with a degree in music and later became a software developer by chance. He founded The Point in 2007, a collective action website from which Groupon was born one year later. After Groupon turned down a $6 billion dollar acquisition bid from Google in 2010, the company filed for an IPO in June 2011. It began trading on Nasdaq in November and has a market cap of $12.41 billion today. Forbes.com: Biggest CEO departures of 2011 Whereas Page and Mason innovated on the web, 39-year-old Kevin Plank provided a new experience for athletes on the field. As a student at the University of Maryland, Plank developed his first business to sell roses for Valentine's Day. It earned him about $17,000, and those revenues later helped him start a company to manufacture moisture-wicking fabric. He was a senior on the college football team when he developed a prototype for a polyester-Lycra-blend T-shirt to keep him and his teammates dry. Fifteen years later he is the chief executive of Under Armour, an apparel company that now has a market cap of $4.27 billion.Forty-year-old workaholic Elon Musk had already been at the helm of several major businesses before moving to his most notable role. When he was in his mid-20s, he and his brother started Zip2, which provided online publishing software for news organizations. After that he cofounded the online financial services and e-mail payment company now known as PayPal. With that solid experience under his belt, Musk moved on in his early 30s to Tesla Motors, the company that builds the Tesla Roadster electronic sports car. He has served as the company's CEO and "product architect" since 2008, and as its chairman since 2004. Tesla today has a market cap of $3.28 billion. Spencer Rascoff is the thirty-six-year-old chief executive of the popular real estate information marketplace Zillow. Before becoming a leader in the real estate industry, the Harvard grad made a name for himself elsewhere. During a five-year stint in the travel industry, he co-founded Hotwire.com and served as vice president of lodging for Expedia. Rascoff joined Zillow in its early stages and held various positions before becoming its chief decision-maker. In 2005 Rascoff served as CFO and vice president of marketing. He then became COO from 2008 through his promotion to CEO in 2010. Today Zillow has a market cap of $915.01 million. Forbes.com: America's 25 highest-paid ceos To make this list, you had to be the chief executive of one of the 20 biggest publicly traded companies in the U.S. (as of Feb. 13, by market capitalization) that have CEOs aged 40 or under. Our data comes from Capital IQ, a Standard & Poor's business. Top 10 Most Powerful CEOs 40 and Under: 1: Larry Page Google Market Cap: $199.05 billion Age: 38 2: Andrew Mason Groupon Market Cap: $12.41 billion Age: 31 3: Kevin Plank Under Armour Market Cap: $4.27 billion Age: 39 4: Andrew Silvernail IDEX Corporation Market Cap: $3.54 billion Age: 40 5 Elon Musk Tesla Motors Market Cap: $3.28 billion Age: 40 6: Mitchell Krebs Coeur d'Alene Mines Corporation Market Cap: $2.42 billion Age: 40 7: Robert Pera Ubiquiti Networks Market Cap: $2.27 billion Age: 33 8: Aaron Jagdfeld Generac Holdings Inc. Market Cap: $2.00 billion Age: 40 9: Edward Rosenfeld Steven Madden, Ltd. Market Cap: $1.72 billion Age: 35 10: J. Mariner Kemper UMB Financial Corporation Market Cap: $1.64 billion Age: 39 Forbes.com Full list of most powerful CEOs 40 and under Forbes.com: Oldest ceos in the world Forbes.com: CEOs with the best and worst online buzz Forbes.com: What makes a great innovative ceo?