What a reactionary zealot response.

This is a dumb exclusionary policy being fairly pointed out, not ageism.

I'll explain it with a comparision of the language used.

The language used for benches like this in my country, Australia: "Priority seating. Please stand for seniors, pregnant, or handicapped people".

Priority, based on needs. Anyone can use it otherwise.

The language used in this sign: "Benches are for seniors are handicapped citizens only"

Exclusionary, based on privilege and needs. If you are old and infirm and a citizen ONLY YOU may ever use this bench, and if you're not a citizen (tourist, asylum seeker, migrant worker) you can fuck off regardless of your needs. Pregnant women? Who cares, they're not old and they're women.

This is not a subtle distinction. This is language used to assert a hierarchy and exclude underclasses, and it's embedded into every aspect of US society without most people even noticing because they've never seen different.