How to make a million before you're twenty

At 15, Catherine Cook and her brother Dave, 17, were flipping through their high school yearbook and came up with the idea to develop a free interactive version online. In 2005, the two convinced their older brother Geoff, a budding Web entrepreneur himself, to invest $250,000 and his time to help them launch MyYearbook.com, a social-networking site based in Skillman, N.J.

Soon after, the Cooks merged with Zenhex.com, an ad-supported site where users post a variety of homemade quizzes, more than doubling the number of eyeballs taking in their site. But when they tried to expand even further, they hit some snags. Potential investors wanted to move the company's headquarters to New York (the Cooks wanted to stay put). They also wanted to have ads appear on users' personal profile pages (the Cooks didn't).

Good thing the Cooks stuck to their vision. By 2006, MyYearbook had raised $4.1 million from the likes of U.S. Venture Partners and First Round Capital. Since then, the site has attracted such advertisers as Neutrogena, Disney (nyse: DIS - news - people ) and ABC; has grown to 3 million members worldwide; and rakes in annual sales in the "seven figures," says Catherine.

How to compete in an industry dominated by MySpace and Facebook? Mine a niche. "[Our site is] specifically for high school students, and we really listen to the suggestions of our members," says Catherine.

While the Cooks decline to discuss the value of their stake in the business, one MyYearbook investor (who agreed to speak only if unidentified) claims the Cooks' chunk is worth "well over $1 million."