So I'm in my forties. It's not that old, honest, and I can stride down the street looking fairly trim. But it's all pretense. You see, I have a smallish daughter (age 7) and when she does these simple tricks on the trampoline or zooms round corners on her Heelys (incredibly cool trainers with roller skate in the heel), I suddenly feel very old indeed. I've tried the trampoline tricks - every one has resulted in my falling 'a over t' and looking extremely silly. And I wish I had the courage to get myself some Heelys - but I just know that I'd again be flat on my back in very short order.
Another example. Some kids on my block were kicking a ball around and it came in my direction. I joined in the kick around and was enjoying it for all of a minute when suddenly I slipped and landed awkwardly - and my 40-year old back put me in agony - this was last summer and it took 2 months for my back to feel normal again.
In my head I feel 25. I'm sure I'm not alone in this mental-physical discrepancy though!!