It's silly to pretend to know why/how someone is "hanging on" unless you're providing medical care, and even then, you're facing intangibles. The most dramatic day in Presidential health alerts was July 4, 1826, the 50th anniversary of the signing of the Declaration, and the death date of our 2nd and 3rd Presidents, Adams and Jefferson. In that case, you could credibly theorize that making it to the great observance kept both men alive in their final illnesses. With Carter, he's got good genes, health habits undoubtedly better than his brother Billy's, longtime commitments to his church and social work -- and, if you reach age 99, you've experienced so much loss that you've learned to put it in perspective and endure. But Jimmy himself probably can't synthesize an explanation for how he's lived this long.
As mere guesswork, I would not discount the proteins, antioxdants, biotin, phosphorus, and the anti-heart disease qualities in magnesium and niacin, all of them found in the humble peanut.