As far as this specific question goes, though, the trope description does seem to make it clear: It's about their deviation from the median age of the cast. In a cast where most of the characters are teenagers, someone in their middle age can qualify (as the trope description specifically says is the case for most anime.)
If the trope is to be salvageable otherwise, though, I do think that middle age has to be the lower limit - in addition to being above the median age of the cast, the character also has to be old enough for their age to plausibly be slowing them down, since some of the meaning of the trope comes from "age isn't slowing them down."
Therefore, even in a cast of toddlers a teenager or young adult can't qualify (outside of odd situations where applying the trope to someone where it shouldn't is Played for Laughs, eg. a teenager in that situation complaining about their aching joints and how they're getting too old for this.)

I’m wondering about that.
dope slaps you* HEY GET BACK TO WORK! We got a dead body