At the risk of being called "whippersnapper", as I'm only 37, I'll chime in. I started training as a child and the injuries have accumulated. Being a martial artist for 28 years coupled with 13 years in the Army have beaten me up pretty well. Still I train, though because it is inherently who I am at this point.
What I notice most is twofold.
1. I don't heal nearly as quickly anymore. I used to be able to shrug off pulled muscles, bumps, and the like in no time. Now it seems if I pull a muscle I regret it for days. I miss being able to bounce back easily.
2. I have lmore patience with a lot of the random martial arts world b.s. out there. I sed to get seriously worked up over the silliest things. Kids wearing black belts, musical kata, gymnastics in karate tournaments. Who got promoted when and by whom, self-promotions, association politics, instructors with ego trips and the like. I have developed a lot of detatchment over the years. I glad for it. How others train and, aside from my seniors and students, how I am viewed doesn't concern me a great deal. I wish I could have understood this 10-15 years ago. I would have been much better as an instructor.
I am hopefull when I read accounts like most of the posts in this thread of guys and gals training for 40, 50, or more years. Hopefully I'll be one of those.
Mark
P.S. I am also hopefull that by the time I have 60 years in the Art that we will have developed Holodecks. The training possibilities are pretty cool.