If we're taking that route, the Constitution itself is already ageist: you need to be at least 25 to be a rep, 30 to be a senator, 35 to be president/VP. So it's already an established idea that age plays some role in fitness for office.

Ageism, like most discrimination, is about writing off someone based on factors outside their control, not requiring someone to mature. Young people eventually meet the requirements to enter government service. Forced retirement based on age is a permanent disqualification.

But you didn't address the existing rationalization: explain to me why the minimum ages exist in the first place, and why that same rationalization shouldn't exist for older people in their more out-of-touch and declining years.

Both ideas can be wrong. The idea that "we already made ageist mistakes in service requirements so we should be allowed to make more" is fundamentally flawed.

I'll wait for actually evidence/data that says that someone who's 32 is incapable of performing job X or that Y% of people over age 60 show marked decline in ability to perform job X before we start endorsing ageist policy.

The electorate as a whole is older (The Boomer lump passing through the population charts), and likely more inclined to keep voting for people of their generation rather than someone their kid (or grandkid!)'s age. Or other reasons. Who knows.

This chart just shows correlation between the age of our Republic and the term length/age of our elected officials.