Whiz kids: CEOs 40 and under of major public companies

Mark Zuckerberg is only 26, and the company he's chief executive of, Facebook, is worth anywhere from $50 billion to more than $80 billion, according to recent media estimates. Andrew Mason, 29, reportedly turned down a $6 billion offer from Google for the company he runs, Groupon, in December. But those are both privately held companies. Who are the CEOs under 40 who run 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. 11, with CEOs 40 and under.

Forbes.com slideshow: America's 20 most powerful ceos 40 and under

They lead a diverse list of companies, including sports car manufacturers, online travel sites, property insurers and electronic learning software makers. Michael Chasen, 38, heads a company that changed the educational experience for millions of students over the past decade. As a high school student, he wrote computer programs for local businesses, charging them $25 an hour for his services. He continued to build computer applications, and when he was 25 he dropped out of law school to start Blackboard, an online learning company. Today it has a market cap of $1.36 billion.

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Whereas Chasen innovated in the classroom, 38-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 $3.56 billion.

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Thirty-nine-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.

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. 11, by market capitalization) with a CEO aged 40 or under.

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