Gates, who dropped out of Harvard to start Microsoft in the 1970s, says that if he were to begin again, he would devote himself to either artificial intelligence, energy or biology.
"The work in artificial intelligence today is at a really profound level, " says Gates, speaking at a Facebook live event broadcast from Columbia University and moderated by Charlie Rose. "It is going to be phenomenal, so anything connected with that I think would be an exciting lifetime career."
The planet needs to find reliable, cheap and clean energy, Gates says, so "the innovations there will be profound."
As for biology, Gates maintains that the coming generations will figure out the science underpinning obesity, cancer and depression.
"In terms of big impact, those three hard science problems would be the top for me," says the man who could become the world's first trillionaire if he lives to be Buffett's age.
Buffett, the billionaire and iconic investor, says that, if he were starting over today, he would make the same choices and become an investor. He started reading about investing when he was just a kid and it's still fascinating to him. (Entrepreneur Mark Cuban had the same answer when he recently faced this question.)
"I had fun when I was in my twenties, my thirties, and now I am 86 and I am having fun," says Buffett.
Also, he says, smiling, he picked what he was good at: "I would be a failure at anything else, probably."
The chairman and CEO of Berkshire Hathaway advises everyone to seek their purpose. "I advise students, as much as possible, look for the job that you would take if you didn't need a job," says the Oracle of Omaha. Doing otherwise, or what he calls "sleepwalking through life," is "like saving up sex for your old age. It's not a good idea."
That last comment makes the audience laugh. But, jokes aside, Buffett says, "You really want to be doing what you love doing." And if you don't connect strongly to your first job or two, "don't give up before you find it."