Around 30, I started to find training for the 800m too intense.
Ran my best 3000m s/c at 35.
Was in good shape turning 40 and started with injury problems which I didn't really overcome until 46 and came back on got third in US Masters 3000m s/c.
Early 50s got some coaching for the first time in a long time, and actually improved from 50-56 and placed in three more US Masters' Champs...
Injured at around 57-58 and had knee surgery (#3), came back and ran 11;32 for 2 miles at 60. Was aiming for World Senior Games and peroneal tendon went a few weeks out.
Took quite a while to get back, but returned to good form at 65 and back on World Rankings for age group. Injured the following year, and now 67 and coming back again.
So, nearly 40 years of highs and lows, but never really out of shape (at most 10 lbs over racing weight, and now back down to same racing weight and same jeans size as always).
Putting that in perspective, the injuries have been frustrating, but getting through them back to good form has been each time has worth it (to me). Overall, running has generally been beneficial health wise (keeping weight done and avoiding meds), and while I'm slower in my running times, in terms of being active, I don't really notice in terms of general activity that I'm old.
I'm fortunate that I'm still engage in my career, and I would say I'm better at what I do than I was 10 years ago, and I get up every morning looking forward to work, which probably helps - I wouldn't do well with retirement.
Overall, even taking the injuries into account, I've been very blessed, and while I'd rather have the fitness I had at 28, and in a lot of ways I feel very much the same way as I did 40 years ago, engaged in my work and focused on the next running goal.