FDR was 63 when he died in 1945.
He'd won re-election in 1944 to his fourth term at the age of 62. (We came too damn close to having Henry Wallace succeed him as President, but that's another story.)
Had he been in excellent health, it's very likely he would have been re-elected again and again. The new Constitutional term limits, assuming they would still have passed, did not apply to a sitting president.
Reagan was 74 when he was elected to his second term. It would not have been impossible for a healthy FDR to have won re-election at the same age, in 1956.
He had two sons in politics.