Jimmy Carter is 99 years old. Carter's grandson says exercise is the key to the former president's longevity.

• Presidential grandson Jason Carter credits exercise to former President Jimmy Carter's longevity.
• The former president turned 99 on Sunday and remains the longest-living president in US history.
• Jason Carter told Time that his grandfather would likely attribute his longevity to his marriage.

Former President Jimmy Carter turned 99 years old on Sunday and remains the longest living commander-in-chief in US history.

And as Jimmy Carter and former First Lady Rosalynn Carter celebrate the momentous occasion this weekend with family and friends, the former president's grandson, Jason Carter, pointed to exercise as the reason for the ex-president's longevity.

"If he got to a new city that he had never been to before, whether there was Secret Service or not, he would say, 'Hey, is there a bike?'" Jason Carter told Time of his grandfather in an interview late last month.

Jason Carter, a former Georgia Democratic state senator and the party's 2014 gubernatorial nominee, told the news outlet that his grandfather consistently made an effort to find places to jog during his many travels and later switched to biking as he grew older.

The former president also previously played tennis daily.

"Stay active," Jason Carter emphasized.

However, Jason Carter told Time that despite his own opinion, his grandfather would likely credit his 77-year marriage to the former first lady for his longevity.

"The best thing I ever did was marry Rosalynn," the former president said during a 2015 interview at the Carter Center in Atlanta. "That's the pinnacle of my life."

Jimmy Carter served as a Georgia state senator from 1963 to 1967 before his stint as governor from 1971 to 1975.

Running for the White House in the aftermath of the Watergate scandal, he won the 1976 presidential election, defeating then-President Gerald Ford, but lost his reelection bid to former California Gov. Ronald Reagan in 1980.

After leaving Washington, the Carters returned to their native Georgia. Since then, the former president has received widespread recognition across the globe for his early efforts to combat climate change and his decades-long humanitarian work.

Jason Carter told Time that his grandfather's tenure in the Oval Office was very much a "millennial presidency," noting that many of the policy items that younger Americans currently want to see addressed were things that the former president foresaw in the 1970s and 1980s.

"If we had listened to his presidency when they started talking about climate change, when they started talking about changing the way that we consume energy, solar and alternative sources — we would be far far ahead of where we are now," he said.

The Carter Center in February announced that the former president was entering hospice care after "a series of short hospital stays" and "has the full support of his family and his medical team."