Some funny comments here. I think age is a mental thing, but there are certainly physical effects. I am only 50, and can still do most of the things I did in 20's. It is just the soreness is twice as much recovery is twice as long. In 20's recovery was a few beers that night and good night's sleep. Now it is the same few beers and Ibuprofen for the next day's soreness
I enjoy working on old cars a hobby. I notice that the garage floor is getting lower and harder as the years progress. Laying on a cold cement floor is much harder on the body than it used to be. So now being older and smarter, i use a piece of cardboard or a piece of old carpet to insulate. Helps the joints tremendously.
There seems to be a very common viewpoint that keeping busy and active is a big part of not having a self-sense of being old. Stop doing things and you become old.