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.