The point of my earlier post, lost in the red mist of outrage, was that there are a number of factors that disqualify a passenger from taking a seat in an overwing exit row, especially the one nearest the exit, but age is not one of them.
Physical handicaps that reduce or remove the ability not only to handle the exit in the right way, fast, but also then to evacuate fast to clear the way is one of those, and obesity is proably the most common and most obvious.
Stupidity, sorry mental incapacity, is another, although quite how you detect it at a glance is not so clear. But I can think of people who would be completely incapable of following the instructions, especially under stress, and have indeed sat next to them in that row and had to explain what the FA was talking about and why, translated into words of one syllable spoken very slowly. One such fit and able 20-something asked me what it was all about, after the CC briefing.
"Don't worry. If we are told to get out fast, you do nothing until I have opened this win-dow. Then I will climb through the hole and you must fol-low. OK?"
"Is the yellow thingy she showed us how to put on so that people can find me?"
"Yes."
"I'll have to get my bag down, though. She made me put it up on that shelf."
"Don't worry, the pi-lot always goes round collecting things up and he'll give it to you after he's gone out through the hole."
"Oh, that's nice, innit."
As I said, age by itself is not a factor.