9. Tumblr: David Karp
At 14, David Karp interned for animation producer Fred Seibert. The next year, he dropped out of high school, later moving to Tokyo to work as a software consultant for online parenting advice site UrbanBaby. In 2007, Karp founded Tumblr, a microblogging and social networking Website, which now has more than 46 million blogs. Today, 25-year-old Karp's net worth is estimated at $400 million.