Let's talk in general terms, not this specific case.
If an old couple checked in and the agent thought they were not mentally sharp enough to sit in the exit seats, a relatively gentle way of letting them down might be to quote an arbitary age limit.
Without wishing to be rude, I've seen some elderly people who were probably fit enough to handle the door, but in reality should not be sitting in that row. In fact I held a meeting in a hotel meeting in Cyprus last weekend and saw several couples who fell in this category.
So are we suggesting that check in agents say "sir and madam, to avoid any implications of age prejudice, I have observed that you appear to be rather slow in your thought processes and speed of reaction, plus your ability to grasp reality as do the rest of us is in question in my mind.
"Therefore, due to my concerns about your mental competence, I have re-allocated to you to 23 D and E."
Do we really wish check in agents to say things like this? If I had elderly relations, I'd rather they were given a white lie.
I suggest anyone pushing the age prejudice agenda thinks hard about that, as you might get what you wish for.