Out of the loop

I think you accidentally an age, but I get your point.

The difference is that we have scientific evidence that demonstrates categorically, without any outliers, that humans below a certain age do not have fully developed brains. On the flip side, we don't have any scientific evidence that necessitates that a human loses their mental faculties after a certain age. Anecdotally, my great grandmother lived to 104, was living in her own house the entire time, and could hold a coherent conversation about the early 1900s no problem (up until the last year or two). In her 90s she legitimately said, "what's wrong with me, all of my friends are dead, why am I still fine?"

Meanwhile, not only has life expectancy been constantly rising over the last few hundred years, but scientists are actively trying to slow or even reverse aging in humans. It's perhaps unlikely, but not impossible for humans to unlock immortality at some point in the near future.

Point being, you can't say anything is necessarily true about all adults after a certain age, just like you can't say anything is necessarily true about /insert race/. So it would be the definition of ageism.

What you really want is some kind of aptitude test to verify that they are still minimally capable of doing their job, but for the same reason that's not used to admit people to vote, you can never be certain that the test doesn't introduce a bias thereby disenfranchising people. So I really think the best we can do is a democracy...something better than first past the post would probably help though.

Edit: btw, i think I see now how you missed an age. Apparently things in angle brackets just get deleted :/