Bill Gates was 19 when he came up with the idea for Microsoft. Michael Dell was the same age when he started selling computers out of his dorm room. Who are the teenagers and 20-somethings trying to hatch the big tech and media ideas of tomorrow? has compiled a list of likely candidates under the age of 21, from web design impresarios to "pimp my MySpace" tycoons.  Taking advantage of the Web's low barriers to entry means that you often only need a really good idea. catherinecook_woCatherine Cook

Age: 19
Company: myYearbook

Some great ideas come from analysis and introspection. For siblings Catherine and David Cook, it was the result of a snarky comment. “My brother David and I were flipping through our high-school yearbook during my freshman year,” Catherine recalls. “We were looking for a girl in his class—I think he liked her—and he was trying to show me who she was. Once we finally got to the picture he was like, ‘She looks nothing like that.’”

The problem, they realized, was that the yearbook photo was a year old. The conclusion: “Yearbooks suck.” The Cooks figured they could make a better yearbook online, one that allowed people to choose and update their own pictures. They got $250,000 from their older brother, Geoff, who had started and sold the web sites EssayEdge and ResumeEdge when he was a student at Harvard.

In less than two months, they finished myYearbook (outsourcing the programming), and launched it in April 2005. By the fall, it was bringing in some 3,000 new members a day. Today, it has more than 9.8 million unique visitors a month from students all around the world, according to comScore.