The fortunes of the youngest 1 percenters have been on the rise, and it's not because they're inheriting their enormous wealth.
And it's not necessarily coming from old-school businesses such as oil and breweries: Only three of the richest people under 40 got their fortunes that way. The majority of the world's richest billionaires under 40 are American, and that's linked to the rise of Silicon Valley-based tech businesses.
The list of young billionaires also has an unbalanced gender split. Women hold only two slots -- which isn't all that surprising given advisory firm Wealth-X's findings that women account for only 12 percent of the world's billionaires. That 12 percent is more likely to have inherited their fortunes, while about 60 percent of male billionaires gained theirs through entrepreneurship.
Most of the young American billionaires either co-founded or have stakes in California-based businesses, which Wealth-X notes is the state with the highest concentration of what it calls "ultra high-net-worth" individuals, or those with at least $30 million in assets.
While still being a select crowd, the billionaire population is on the rise, thanks to the surging valuations of stocks, real estate and other assets during the past several years. In 2014, the ranks of billionaires grew 7 percent, with 155 more people joining the club. The combined value of the assets held by the world's 2,325 billionaires is $7.3 trillion, or more than the market capitalization of all the companies in the Dow Jones industrials index.
Read on to learn about the nine richest billionaires under 40.