Competitive athlete until 36 and then could no longer train with volume and intensity that I could when I was in my 20's and early 30's. I'd say physically I hit my peak at 32 from the mental hardness that I had developed to take extended amounts of pain and keep coming back for more.
At 36 after training hard and competing I felt like I wanted to sleep for days afterwords. I knew it was over.
I think you can still do high intensity for short periods as you age, but you have to keep those periods short, make them count and allow yourself plenty of rest. Training becomes more methodical to avoid injury as you get older.
Also I think you get tired of being banged up and older, your mind is tired of going down that road. Also mentally you've known what it was like to be in the best shape of your life, there are no unknowns, you've been there and done it. Perhaps that makes it so you don't care near as much about going there again when you're older.