Patrick and John Collison

The Irish-born brothers are a couple of the world’s youngest self-made billionaires. Patrick and John dropped out of MIT and Harvard, respectively, and sold their first business—an online auction management company—for a reported $5 million in 2008. Then they embarked on the idea that would become Stripe, now a payments behemoth valued at $9.2 billion. The company processes tens of billions of dollars in digital commerce transactions each year, inking deals with the likes of Amazon, Lyft, and Facebook. (Also No. 17 on the main Fortune 40 Under 40 list.)