For me, it was about age 38... had been racing at a high level since 1980 (at age 15) and suddenly, with no real difference in training, diet, sleep... the legs got less resilient, more nagging injuries that seemed never to go away... The race times had stagnated or gotten slower, even though the workout times were still consistent and fast.
Once I hit 40 I started actually mentally accepting that many of my PRs were behind me and that I had to change the dynamics of my running, if I wanted to stay competitive. I began stretching more regularly, and not just the same stretches learned in high school PE. I did some more weight work and core work to balance out the muscle groups so long ignored. I reduced my racing, and planned my training more specifically to my life schedules.
It has worked to the extent that I am back running with more liveliness in my legs, not feeling injured all the time, and back to training at sub 6:00 pace. Can't wait to get my spring and summer racing under way. Still thinking I can try to win at the local road race level. Times will tell.