Some people also young better than others, though. There are 18 year olds in the world I've have no problem voting for, if I could.
But yeah, a lot of age limit sentiment seems to be just straight up just ageism to me, as if every person becomes senile as soon as they turn 80, or even just 70 or 60, which just isn't remotely true. Intelligence can remain sharp as ever, and sometimes elderly wisdom is indeed a thing. And every politician is surrounded by aids who will notice if something starts to go wrong.
I'd be sort of okay with a very high age limit, like 90, I guess, but on the whole I agree term limits are better anyway.