The billionaires have organized the "Giving Pledge,” an initiative launched five years ago to convince others to donate at least half of their wealth to charitable causes around the world.
Buffett, 84, who has pledged to give away 99 percent of his nearly $73 billion fortune during his lifetime or in his will, told ABC News’ Rebecca Jarvishe asks prospective donors to put their money to good use.
"I just say, 'Hi, Rebecca ... what are you thinking about doing with all that wealth that you've got?'" Buffett said in an interview for "This Week." "You're not going to live forever.
"The money has great utility to other people," he added.
Over the past five years, Buffett and Gates have convinced 133 others to sign on to the Giving Pledge. "We had no idea we'd get this many people to come together," Gates, 59, told ABC News.
Both try to convince billionaires to spend their money as early as possible. "The sooner the better. We're always encouraging people that it's fun," Gates said.
Chobani yogurt founder Hamdi Ulukaya, a recent signee to the pledge, will put his money toward a charity helping refugees around the world.
"You can save life, you can make somebody's life better," Ulukaya said. "Why would we put it somewhere and wait for 50 years, and 70 years while you can do it right now?"
Or, as Buffett puts it, "If I get some 75-year-old saying he'll think about it later, I'll ask him, 'Do you really think you can come to a better decision when you're 95 with a blonde sitting on your lap?'"