Mark Zuckerberg is NOT the world's youngest billionaire.

This distinction actually goes to his former roommate, Dustin Moskovitz! Moskovitz is 8 days younger than Zuck, and has a net worth of $1.4 billion. Only eight American billionaires are under the age of 40, and three of these co-founded Facebook: Zuckerberg, Moskovitz, and Eduardo Saverin. The latter two are no longer with the company, however - Moskovitz left in 2008 to start his own software venture, and Saverin was sued by Facebook a year after Zuckerberg dropped out of Harvard. Though Zuckerberg is no longer the youngest billionaire in the world, I'm sure he's not fretting one bit, considering at $6.9 billion he is still worth nearly 5 times as much as his college roommate!