At 22, Luca Yang was running a small teaching company in New Zealand after quitting a six-figure finance job that he hated. It had humble beginnings, where he tutored English online for A$25 (US$17.55) per hour just to get by. When one of his students got into Oxford, word spread: one student became 50, then 500. By most measures, he had already won the start-up lottery. “It was producing a lot of money, but it wasn’t something I actually loved doing,” says Yang, who was born and raised in...