Who is the richest person you have known? How did they build their wealth?
The first person who comes to mind is a peer of mine from my early twenties. I may have known wealthier people over the years, but considering his age I'd say he wins. He didn't flaunt his wealth but was obviously a millionaire many times over by his early twenties and earned all of it. You couldn't fake his level of living through consumer debt. I once attended a party at his mansion in Chicago, and I remember a lot of spoiled rich wives at the party. He sort of politely laughed at his wife's teacher salary, and hoped she would one day get a real job. This guy was a professional futures trader. He worked for large companies who entrusted large sums of money to his care, and he made it grow by investing in commodities all day long. He had done it since his teens and obviously loved it. Having toyed with trading commodity options already, I once asked him how a person learns to be a successful trader. He replied that yes a person can learn, but really shouldn't try it until he can afford to lose about $100k. I was probably making $6k/year at the time, so we may as well have been discussing how large is the sun. He retired at maybe 26 and now is a philanthropist. Good guy, solidly nice and trustworthy all the way. His age and skill level, and what he did with it, left an impression--of what I *should* be doing with my life.
I've also known a billionaire heiress (who was insane)--granddaughter of the founder of what is now a major public company.
So, business and real estate.