Age and the presidency
By 
The current and likely presidential candidacies of Bernie Sanders and Joe Biden provide an occasion to discuss how age ought to be taken into account when considering potential presidents.
Some fun facts:
Of the 45 U.S presidents to date, only three were older than 65 when taking office. These three instances aren’t exactly an advertisement for the advisability of electing someone in his late 70s. (Sanders would be 79 when inaugurated, and Biden would be 78).
William Henry Harrison was 68, and famously dropped dead within a month. Admittedly that was a long time ago, but . . .
Ronald Reagan was 69 (a full decade younger than Sanders/Biden would be on taking office). Reagan was clearly suffering some serious mental decline during his second term: so much so that there was talk among his aides about potentially invoking the 25th amendment.
Donald Trump was 70. Trump has always been an utterly loathsome grifter without a single redeeming characteristic, but if you watch interview with him from 20 or even 10 years ago, his present mental decline is striking.
This is a very small sample of course, but OTOH it’s the most important job in the world, and there’s not exactly a shortage of qualified applicants, although due to the Wisdom of the Framers none of them currently occupy the office.
A word about ageism. To be a coherent concept, ageism means taking someone’s age into account in an inappropriate way, not merely taking it into account at all. There are all sorts of reasons to be highly skeptical of electing very old people to the presidency.
First, the risk for dementia doubles every five years from age 65 on. It is very low at first, but by 80 it’s getting quite significant, and by 85 it’s a big risk.
Second, electing an 80ish president means electing someone whose initial and therefore in many ways most important formative political experiences were literally two generations ago, i.e., in the 1950s, aka a radically different world.
Third, leaving aside more drastic forms of cognitive decline, really old people are generally not as good at learning things/changing their minds when doing so is warranted than they were when they were younger.
Of course there are always going to be individual exceptions to all such generalizations, but my rule of thumb is, look, why take a chance?
At least that’s the way I feel about it.