This year there are 20 American billionaires are under the age of 45. The youngest member is Dustin Moskovitz. The 29-year-old Facebook cofounder has been the youngest billionaire in the country since he first appeared on the list in 2010. This year his net worth has nearly doubled to $5.3 billion, thanks to his 5% stake in the ubiquitous social network.

Mark Zuckerberg, the Facebook CEO, comes in as the second-youngest FORBES 400 member, just 8 days older than his former Harvard roommate Moskovitz. Zuckerberg, however, has nearly 3 times as large of a fortune. His estimated net worth is $19 billion, up from $9.4 billion last year as the stock returned to its IPO heights. That’s good enough for him to rank as the 20th wealthiest person in the country.

Even beyond the Facebook duo, Silicon Vally dominates the youngest billionaire ranks. Napster cofounder and former Facebook CEO Sean Parker is 33, while Twitter cofounder and Square CEO Jack Dorsey is 36. 

Meanwhile, you can’t forget about the Google, Sergey Brinand Larry Page. The 40-year-old cofounders have a combined net worth just shy of $50 billion, which would be the 3rd-largest fortune in the United States.  Forty-two-year-old Elon Musk also sits among the youngest and wealthiest. The Tesla CEO’s net worth jumped to $6.7 billion this year, up from $2.4 billion last time; in percentage terms he was the second biggest gainer of the year.

But you don’t have to be a tech maven to make this list. Under Armour founder and CEO Kevin Plank, 41, carries the flag for more traditional retail businesses. 

Also, Milane Frantz is the only women among the youngest 20. Frantz, 44, was one of four heirs (including her younger brother Scott Duncan) of Dan Duncan‘s energy pipeline fortune after he passed away in 2010.

A separate survey also released this week analyses theyoungest generation of millionaires. The Fidelity Millionaire Outlook shows that rich Gen X and Gen Yers are earning more money and amassing more assets than baby boomer millionaires