Most people are fixated on how old other people are. They want to pigeon hole everyone by age. Just look at any newspaper report.
"Mrs Jones, 73, said she saw the cat stuck up the tree."
"Mr Smith, 28, said he was stuck at the traffic lights for forty five minutes."
Immediately we form an idea of what the person looks like. Mrs Jones has permed, white hair, wears a blue anorak and walks with a stick.
Mr Smith is just an average man.
However, if Mrs Popbottle, aged 73, saw the cat, she would be expected to look rather like Mrs Jones, but Mrs Popbottle is fit and slim and is a practising 5th dan black belt in Shotokan karate and Mr Jones is just an average man, if there is such a thing.
So we make judgements about people when we know their age. Our expectations are different for different age groups. That doesn't matter too much if you're judging someone else, although it perpetuates the assumptions, but we do it to ourselves! We tell ourselves, "People of my age don't do certain things..." So they don't do them even if they would like to!
My mother-in-law used to ask me (virtually every time I saw her),
"Are you still doing karate, Liz?"
The implication being, " Surely, at your age you should have stopped doing that or at least be incapable of it! "