Each year, Forbes crunches the net worth of every American billionaire to see who will make the exclusive Forbes 400 list. But 11 members of this club are luckier than the rest: they are in good health, have full heads of hair and many, many years to spend their fortunes.

This year there are 11 American billionaires under the age of 40 on the list. The youngest member of the Forbes 400 is 30-year-old Facebook cofounder Dustin Moskovitz, who has been the most junior person on the Forbes 400 since making his debut in 2010. He lost bragging rights as country’s youngest billionaire this summer when Snapchat raised money valuing the photo sharing app at $10 billion. Now Evan Spiegel, 24, holds that title but with a net worth of $1.5 billion, he just misses the cut for the Forbes 400.

Moskovitz is just eight days younger than his former Harvard roommate, Mark Zuckerberg, who is the second-most-junior member of the list. The Facebook CEO is worth an estimated $34 billion, up from $19 billion last year, making him the year’s biggest dollar gainer. At just 30 years old, he is the 11th richest person in America – and the wealthiest under 40.

read more >>>Youngest Richest Americans 2014

Mark Zuckerberg’s roommate at Harvard and Facebook’s third employee, Dustin Moskovitz left the social network in 2008 to start software firm Asana. He and his wife, a former Wall Street Journal reporter, devote some time to their Good Ventures, which has given millions to such causes as malaria eradication and marriage equality. Moskovitz bikes to work and flies commercial. He is also a regular attendee at Burning Man.

A close third is newcomer Elizabeth Holmes, also 30 but born in February, who joins the ranking this year thanks to her blood testing company Theranos. Holmes founded the Palo Alto, Calif.-based company with money saved for college, and has reportedly raised more than $400 million, valuing the 10-year-old company at $9 billion . Two other new additions to the Forbes 400 who fall into the under 40 club are WhatsApp’s Jan Koum, who, aged 38, is worth $7.6 billion, and Uber cofounder Travis Kalanick, also 38, who joins with a $3 billion fortune thanks to his ride-sharing company.

Unsurprisingly, Silicon Valley whiz kids dominate the billionaire nursery. Napster cofounder and former Facebook CEO Sean Parker is 34, while Twitter cofounder and Square CEO Jack Dorsey is 37. Wireless network entrepreneur and Memphis Grizzlies owner Robert Pera, 36, ranks with a $2.8 billion fortune founded on his Ubiquiti Networks while GoPro camera maker’s Nick Woodman, 39, is worth an estimated $3.9 billion. In percentage terms,Woodman is the year’s biggest gainer, with a 200% increase in his net worth since last year, to $3.9 billion. GoPro went public in June; the stock has been soaring as more people buy GoPro cameras and strap them onto bike helmets, surfboards, scuba gear and more.

Outside of tech, there are still fortunes to be made – or inherited. Scott Duncan, worth $7 billion aged 31, is one of four billionaire siblings who are heirs to the energy-pipeline fortune built by their late father, Dan Duncan. While hedge fund managers have previously had a strong showing in the freshman class of the Forbes 400, this year only Chase Coleman, 39, makes the cut. His Tiger Global Management has about $12 billion in assets under management; Coleman’s personal fortune nears $1.9 billion.

This year’s group of 11 youngest billionaires in America have a combined net worth of $78.5 billion, or just under 4% of the $2.29 trillion aggregate net worth of the Forbes 400 ranking.