9 richest billionaires under 40

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.

9. Brian Chesky, Airbnb: $3 billion

Brian Chesky, 34, started his career as a designer for an industrial design firm after he graduated from the Rhode Island School of Design in 2004. Later, he founded Airbnb with Joe Gebbia, whom he had met at RISD. At the time, they were broke and wanted to earn some extra money by renting out some sleeping space in their apartment to conference attendees.

The idea sparked what's now a global company with a $25 billion valuation, thanks to investors such as Tiger Global Management and private equity firm General Atlantic.

"It's not natural for someone like myself to be at art school, to then be unemployed, and then five or six years later have this," Chesky told Fortune magazine earlier this year. "Nothing really prepares you for that."

8. Nathan Blecharczyk, Airbnb: $3 billion

Nathan Blecharczyk, 32, joined Chesky and Gebbia as the third co-founder of Airbnb when the trio debuted the site as Airbedandbreakfast.com. With a background in computer science, Blecharczyk took on the role as chief technologist, and the three designed the initial site in just three weeks, according to Wealth X.

He started writing code when he was 12, and was earning money from doing that when he was only 14. After graduating from Harvard, Blecharczyk worked at companies including Batiq and Microsoft (MSFT), before joining Airbnb in 2008.

7. Elizabeth Holmes, Theranos: $4.5 billion

Elizabeth Holmes, 31, earned her fortune through her medical startup, Theranos. Holmes, a Stanford dropout, has drawn comparisons to Apple (AAPL) co-founder Steve Jobs, thanks to her uniform of a black cotton turtleneck and black pants.

Like Jobs, she's also no stranger to controversy. While her company has earned a valuation of more than $9 billion, some are asking whether that's overvalued, given questions about the the company's technology, which promises to require only a tiny prick of blood to deliver medical test results. Drugstore chain Walgreens (WBA) recently said it won't roll out new Theranos blood-testing centers until some questions about the technology are resolved.

6. Scott Daniel Duncan, Enterprise Products Partners: $5 billion

Scott Daniel Duncan, 32, isn't as well known as some of the other names on this list, partially because the source of his wealth is an inheritance left by his father, the founder of oil company Enterprise Products Partners.

Scott Daniel Duncan's fortunes were helped by a tax loophole: His father died in 2010, a year that Congress allowed the estate tax to lapse. If he had died a few months earlier, his estate would have been taxed at 45 percent.

5. Eduardo Saverin, Facebook: $5.3 billion

Eduardo Saverin, 33, co-founded Facebook with Mark Zuckerberg, a fellow Harvard student, and served as its CFO in its early years. But when Facebook moved to California, Saverin remained on the East Coast.

That created some hard feelings, according to Business Insider, especially after Saverin froze Facebook's bank account. Zuckerberg cut back Saverin's stake in the social network, according to a memo obtained by the publication. Not surprisingly, Saverin sued Facebook for breach of fiduciary duty, and was given 5 percent of Facebook in a settlement, according to media reports.

Saverin garnered some negative attention when he "unfriended" the U.S. by renouncing his citizenship before Facebook's IPO. That move may have helped him reduce his tax bill.

4. Yang Huiyan, Country Garden Holdings: $5.9 billion

Yang Huiyan, 34, keeps out of the limelight, according to Wealth X. Her wealth stems from her father, who in 1992 founded Country Garden Holdings, a property developer and hotel operator.

Although she grew up in China's Guangdong province, Yang attended Ohio State University and later joined Country Garden Holdings in a number of management roles. She is believed to have inherited her father's stake in the company before it sold shares to the public in 2007.

3. Dustin Aaron Moskovitz, Facebook/Asana: $9.3 billion

Dustin Aaron Moskovitz, 31, co-founded Facebook with Harvard roommate Mark Zuckerberg, and moved to California when the company relocated there. Like Zuckerberg, he dropped out of Harvard to focus on building Facebook.

Moskovitz didn't stay at Facebook for long. He left in 2008 to start a company called Asana, which is a work-productivity application. He has also made investments in startups including FlipBoard and Path. Pledging to give away all his wealth, Moskovitz has opened a foundation called Good Ventures, which is run by his wife, former Wall Street Journal reporter Cari Tuna.

The couple is considered one of Silicon Valley's "most low-key billionaires."

2. Alejandro Santo Domingo, Quadrant Capital/SABMiller: $15.9 billion

Alejandro Santo Domingo, 38, manages his family's money through Quadrant Capital, an investment firm based in New York, according to Bloomberg. The Harvard graduate's wealth stems from his family's 14 percent stake in SABMiller, one of the world's largest brewers, as well as investments in companies such as Chile's Corpbanca.

1. Mark Zuckerberg, Facebook: $41.6 billion

Mark Zuckerberg, 31, started Facebook with some of his fellow Harvard students, and dropped out when he was a sophomore to focus on the social network. His wealth is tied to his stake in Facebook, rather than his earnings as its chief executive, for which he gets a salary of just $1 per year.

Zuckerberg, who turned down a $1 billion takeover offer from Yahoo (YHOO) in 2006, is involved in more than running Facebook. With his wife, pediatrician Priscilla Chan, he's one of the country's most generous philanthropists, according to the Chronicle of Philanthropy.