It's doubtful Kennedy would have lived that long tbh given his health.
He's have been 67. Improvements in medicine could have kept him alive that long. The last President to die in office under the age of 60 who was not assassinated was Warren Harding (age 57). Prior to that, James Polk died at age 53, months after leaving the White House.
Of the Presidents who died in their 60s, FDR was a heavy smoker with a history of polio who was, truthfully, too sick to run for a 4th term (and, likely, too sick to run for his 3rd term). Coolidge died at age 60 of a heart attack; he was a clean liver with cholesterol issues. TR died at 60, but he had health problems stemming from an enlarged heart due to his childhood asthma, malaria, a 1916 gunshot wound, and other health conditions that came about from his strenuous lifestyle. Grant, a heavy cigar smoker, died at age 63 of cancer. Andrew Johnson, an alcoholic known as "Andy The Sot"' died at age 66 after returning to the Senate in 1875. Franklin Pierce, age 64, was a depressed drunkard. Zachary Taylor supposedly died of a stomach disease from being poisoned by drinking water at age 65, but there have been theories that he had been poisoned, and they have not been definitively disproved. William Henry Harrison died at age 68, also from poisoned drinking water (not from his bad cold).
So who knows if JFK would have made it. Likely, he would have. I suspect, however, that had he made it, his health would have been a major issue.
It would have, I could see it sort of being an issue on both sides. His health vs Reagan's age. Who knows who would have come out on top. Would definitely have made for an interesting matchup.
