He's probably not going to be a 2 termer but age is a very subjective thing, FDR died in his office at 63, lost the movement of his legs etc but was still kicking ass in the 1940's !
FDR and Bernie Sanders aren't even comparable. President Roosevelt was a formidable politician, not to mention he was relatively young and very charismatic when he was elected President. On top of that, the U.S. was in the worst economic situation in history, and FDR appealed to both the left and the right in 1932. Sanders? Not so much. I've heard the comparison of FDR and Sanders multiple times, and really, it's a bunch of malarkey.
As for their health, FDR's polio barely affected him, plus the press managed to keep it secret. It was the classic presidential aging combined with the largest war in history that caused his health to go under. 99% sure Sanders is not immune to presidential aging.
Hillary was younger than Bernie & in much worse shape, could barely campaign continously (needed rests, fewer stops, this health condition or that).
You have a good point here, but we don't know how Sanders will be when he's two years shy of being eighty.
There is barely a prominent Democrat who can keep up with Bernie even now with his ridiculous schedule & rallies - As a matter of fact most young journalists in their 20's & 30's were struggling to keep pace with him (he was doing 6-7 rallies a day sometimes in even 3-4 different states).
And he did 14 rallies in 12 states for Hillary in the last 5-6 days running coast to coast. If he is in around that kind of health, that he can travel like mad & campaign, damn right he will run! Jimmy Carter is 92 & is in decent health !
This is a bit subjective. There were no substantial Democratic candidates in 2016 besides him and Hillary. I can guarantee Booker, Gillibrand, and Cuomo could have the power to do more than that if they choose to do so. I'd like to see a source on these claims. Also, Carter's health is irrelevant. He hasn't been in elected office in nearly four decades.
And Reagan had Alzheimer's right which is why he started to decline? But he still got more stuff than any Republican President in history for the "Conservative ideology". As a matter of fact, FDR & Reagan are the 2 most successful (atleast for their parties) & transformative presidents who brought a re-alignment & both were pretty old compared to their times !
Reagan had Alzheimer's at the end of his term when he was 77, one year younger than Sanders if he were to take office in 2020. You keep using these presidents who were decades younger than Sanders in 2020 who had health problems at the ends of their presidencies. There's a lot of holes in this argument.