This time helping catapult two new 20-somethings into the ranks of America's richest for the first time and bringing down the average age of America's richest ever so slightly to 65.7. Only eight American billionaires are under the age of 40, and three of them cofounded Facebook.

The youngest? It isn't who you think it is. The world's youngest billionaire is now 26-year-old Dustin Moskovitz, who is 8 days younger than his former Harvard roommate and Facebook cofounder Mark Zuckerberg. The social-networking site's first chief technology officer, Moskovitz left in 2008 and started Asana, a software company that allows individuals and small companies to better collaborate. The company has attracted several of the same early backers as Facebook and may one day be worth something. For now, though, Forbes estimates Moskovitz's entire $1.4 billion fortune comes from his 6% stake in Facebook.
Zuckerberg may no longer reign as the youngest member of the Forbes 400, but he has bragging rights as the year's biggest percentage gainer; his net worth jumped to $6.9 billion, up from $2 billion, making him worth nearly five times as much as Moskovitz and more than even Steve Jobs.