@ossobuco,
I think I understand what you mean Osso. I have noticed a real sort of sea change in my father's affect as he gets older. He's always been a very take-charge, hale and hearty, competent and happy person with an outgoing demeanor. I've noticed as he ages and one after the other of his parents and his older brothers have aged before him and died, he is quieter.
Not unhappy or depressed- just quieter. And I think he does think about the fact that what once looked like unlimited future (and I know no one's future is unlimited by guarantee whatsoever - but if you're reasonably healthy you might think you will have a certain number of years to enjoy life) is now given some definite limits.
Because if you have enjoyed your life thus far - just because you're older, doesn't mean you're ready to see it end.
I have an 87 year old friend who says almost everyday - when I ask him how he's doing: 'Fine - I just wish I were ten years younger.'
He doesn't want to be young again - he just wants to know that he'll have another ten years.
I think if I'm healthy - I'll feel the same. It'll be hard to think of trading any of my years. Because I have really enjoyed life and I won't want it to come to an end. I can't say what I'll think or feel if I'm not healthy.
The good thing about life though is the fact of the matter is that if you add life to your years you also tend to add years to your life.