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21-year-old billionaires in the making? To tell the truth, the youngest Forbes has come up with in the past decade was Elon Musk at 27. That was back in 1998, with only $22 million. Musk's face is more lined, but he still isn't a billionaire, even after cashing out from PayPal's sale to eBay. Forbes at least has some standards — only reason I can imagine Zuckerberg isn't in the piece is because his share of Facebook's valuation is still mostly theoretical. As for Bebo's Michael and Xochi Birch? They're back to their birthday announcement and e-card concern BirthdayAlarm.com, not content with a cabin in the hills at all. (Photo by Ryan Anson/Bloomberg News/Landov)