I'm 41 and I do occasionally feel it. I don't have any major chronic illnesses unless you count gum disease (which only really bothers me at the perio's office). But here is a trick I have tried which may sound corny but which does help my attitude when I start feeling old...

Pretend you're a time traveller. You're actually 80 years old, your body has pretty much let you down, you can't see very well or get around much at all. Suddenly, you get to spend one day being 43 again. Think of all the things you can suddenly do again!

Oh well, that probably just sounded weird and trippy. But it works for me. Sometimes. :P