What’s the difference between ageism and a prudent regard for statistics?

This very-much-not hypothetical question is raised in my mind by Scott’s observation earlier today that Bernie Sanders is at this point one of the two leading contenders (along with Kirsten Gillibrand) for the Democratic nomination in 2020.

I’m far from a Bernie-basher and in fact preferred him to Clinton in the 2016 primaries, but the man will be 80 in 2021.  Prevalence of Alzheimer’s and related conditions by age:

There are also rumblings that Joe Biden (79 in 2021) is considering a run.

And of course dementia is just one of many strongly age-related health risks, although it’s one that seems especially germane at the present moment, given Trump’s staggering incoherence (he does seem to have gotten worse relative to interviews he did 10-15 years ago).

Anyway, this seems to me to be a difficult issue in a number of ways.