How can anybody know if he's actually too old? Maybe he'll be totally lucid, everybody's different with that kind of thing. He seems pretty vigorous to me. I get it, he could 'die of old age', or something. Anybody could die with little warning, Trump's old as fuck too and is also obese, hasn't stopped him. If he is too old that'd become clear during the early campaign, it's not like Sanders would sit down in the oval office on day 1 and say 'oh shit I'm 78 years old, where are my keys *dies*'.
I think people are just trying to pre-empt voters looking at him and thinking 'wow what an old fucking man' and not voting for him, because some voters will be that stupid, yeah. So to be on the safe side I'd find somebody similar to Bernie Sanders with the same capacity to motivate the left and the youth vote, but who is <70. That person might not exist but it's still a few years away you never know. Nobody had any idea who Bernie Sanders was 4 years ago either.