1 in 10 college students in the US is 40 or older

That’s not relevant - the schools in the area do not have undergraduates in their middle age.

Middle aged college students usually attend extension schools part of the university, but not part of the undergraduate college.

In any case there’s no strict limitation on being an undergraduate at say 60 (I believe Harvard Colleges oldest graduate was in their 80s), but it’s so exceptionally rare it’s ageism.

There’s no reason why one would need to be an undergraduate.

Why not?

That’s simply their rule.

Where is this rule? Even in your other reply you say there's "no strict limitation" so it's not a "rule", just a trend.

Dorm rules mainly. Like I mention in a sibling comment, given the demographics and the arbitrariness of the decision it’s effectively ageism without being explicit.