6. Mark Zuckerberg
Still in his 20s, Mark Zuckerberg, 29, is the third youngest to make Forbes list of billionaires. The co-founder and CEO of Facebook's net worth jumped in the past 12 months, landing him on the 21st spot on Forbes' list, up from No. 36 in 2012, thanks in large part to doubling the company's shares in the past year. Zuckerberg is no longer the youngest on the Forbes list. That distinction goes to 24-year-old Perenna Kei, who owns an 85 percent stake in her family's Logan Property Holdings company.
Forbes Rank: 21