Warren Buffett’s son Peter didn’t find out his father was a billionaire until he was in his 20s—and he learned it the same way the rest of the world did: by spotting his dad’s name on a list of the richest Americans. “I’m not kidding. It was when I was in my 20s that my mom and I talked at some point, because there he was, on this list,” Peter, now 68, recalled in a 2013 Forbes interview with both Peter and Warren Buffett. “And we laughed about …