Jimmy Carter became the oldest living former president in American history when he turned 95 on Oct. 1.
"It's hard to live until you're 95 years old," Carter told People magazine recently, while building homes in Nashville for Habitat for Humanity.
And he attributes his longevity to his relationship with his wife of 73 years, Rosalynn Carter.
"I think the best explanation for that is to marry the best spouse: someone who will take care of you and engage and do things to challenge you and keep you alive and interested in life," Carter told People.
The advice echoes that of 89-year-old billionaire Warren Buffett. "You want to associate with people who are the kind of person you'd like to be. You'll move in that direction," Buffett said during a 2017 conversation with Bill Gates at Columbia University. "And the most important person by far in that respect is your spouse. I can't overemphasize how important that is."
He gave similar advice at the 2009 Berkshire Hathaway annual meeting: "Marry the right person. I'm serious about that. It will make more difference in your life. It will change your aspirations, all kinds of things."
Buffett was married to his first wife, Susan, from 1952 until she died in 2004. He has been married to second wife Astrid Menks since 2006.
As for Carter, he has said that he knew he wanted to marry Rosalynn after their first date, which was a trip to the movies. They started dating while Rosalynn was a freshman in college and Carter was home for a visit while attending the U.S. Naval Academy at Annapolis.
The pair married in 1946, and Carter went on to become the 39th president, serving from 1977 to 1981.
These days, philanthropy is a big part of the Carter's relationship, and they've been working with Habitat for Humanity since 1984, building and repairing thousands of homes with the organization. They're also avid bird-watchers, and told People that they enjoy tennis and downhill skiing.
"I think both mine and Rose's minds are almost as good as they used to be, we just have limited capability on stamina and strength," Carter told People. "But we still try to stay busy and do a good job at what we do."
They often spend birthdays or special occasions alone at home, and keep phone calls and emails to a minimum, he said.
The Carters are also known for living frugally. They live in the Plains, Georgia, ranch house Carter built in 1961, which was assessed at just $167,000, the Washington Post reported in August 2018. Carter also shops for clothing at the Dollar General and flies commercial.
"It just never had been my ambition to be rich," Carter told the Post.
Like this story? Subscribe to CNBC Make It on YouTube!