I'm 43, and have had several noticeable years where things changed and got tougher. Just to be clear, I've always stayed in shape, although I haven't been a runner through it all. I ran in HS, played soccer in college, cycled until my late 30's, then started doing triathlon.
I felt noticeable changes in my body 3 times, at 19, 28, and 41. At 19, I got noticably stronger, but noticed that my recovery wasn't what it was when I was 16 or 17 (I ran 70 a week all through HS). I know that may sound funny, but in my case it was true. The worst however was when I turned 28. Huge drop in recovery time with no increase in strength. From 28 till 41, I basically felt the same. At 41, something changed and again recovery time increased.
However, my strength is WAY up over what it was 20 years ago. I could have never done an ironman in my 20's, but at 40 something I can swim balls out for an hour, average almost 23 mph on a 112 ride, and still manage to run 7 something per mile for a marathon. No way I do that in my 20's. I'm also a lot smarter, and troubleshoot my injuries much quicker. I just wish I could recover from a hard WO in a day. Now it takes 3. Sucks.