...it's the sudden stop at the end.
I remember the first time I realized my rules had changed. I was watching a chase scene in in a movie. The good guy was chasing the bad guy (or vice versa -- I don't remember) and they kept climbing up walls and fences, then leaping down on the other side. Every time one of them landed, I silently flinched. I could almost feel the pain in my knees and back.
I don't know whether we gain wisdom with old age...
...or just aches and pains.