There's a very simple reason why billionaire investor Warren Buffett has been so successful, according to "Psychology of Money" author Morgan Housel. It's because he's been in the game for so long, Housel told CNBC's Bob Pisani on Tuesday on the sidelines of the Future Proof conference in Huntington Beach, California. In fact, "99% of Warren Buffett's net worth was accumulated after he was 65 years old," he said. "The real lesson, if you look at his net worth, is that it is just the amount of time he's been doing it for," Housel added. "If Buffett had retired at age 65, you would have never heard of him." The takeaway is that investors should change their mindset. Most think about how they can earn the highest returns, when in fact they should really be thinking about the best returns that can be sustained for the longest period of time, he said. Buffett, who is worth more than $97 billion , recently turned 92. The Berkshire Hathaway CEO has spent the past year expanding his company's energy empire . Watch Pisani's full interview with Housel in the above video.