At 14, Pari Singh made just enough money to stir up an existential crisis. 17 years later, Singh still can’t talk about the details of that business (at least, not to a reporter). But the deal kicked off a seminal episode in his life. “It wasn’t a lot of money, but enough that I thought I’d sold out,” said Singh, who grew up in London. “I was 14 and I had to work out what I wanted to do with my life. So, every day for two weeks, I thought about it really deeply. I did nothing else. And I basical...