I assumed the joke was that as you get older things are just more stiff and you don't recover as fast. Yeah, it can be reduced with more exercise/activity but you're still getting older.

There was an event where this became apparent for me. I played softball for years in my 20s. Stopped for a bit and returned in my 30s. I was actually in better shape when I returned. One day mid-season, I was rounding first base, not even particularly fast, and I felt something tweek. By the end of the day I was stuck on the couch and could barely move. I had to take the next day off.