In almost any other occupation I wouldn't worry about 78 being old.
I wouldn't even worry about 78 with a mild heart attack being too old.
With the office of POTUS however, the consequences of being sidelined by something that serious, are wide ranging and can potentially affect every man, woman and child on Earth.
So it's not that I think Bernie is too old, I think he's too old and frail, only for the job of POTUS. I think that he should finish out as many terms in the US Senate as he wants.
I even think that he should be a valued member of the incoming POTUS cabinet.
I'd even be okay with him being VP if paired with the right candidate.
And if by some miracle Bernie DOES WIN the Democratic Party nomination for POTUS, I WILL vote for him in November 2020, but unlike 2016, I won't support him in the primaries like I did last time around. My support goes to Liz Warren, who is clearly healthier.
It's not ageism...the man just had a heart attack.
I might (God forbid) have one just like his tomorrow, but I wouldn't dream of running for POTUS in the aftermath if I did.
If he gets the nomination, I will cast my vote for Bernie 2020 as POTUS but he is not my first choice and I would hope he would pick a more robust running mate, and I would worry about him surviving the job every single day he was in office.