I'm 65 and have been running for and racing for more than 50 years. Was a British club runner until late 30s, then moved to US and mostly road-racing, with the occasional foray on the track for USATF Masters races.
Kept the intensity going most of the time - ran very near to my fastest 3000m s/c at age 35.
Had a knee surgery mid-40s, but got back quite quickly and was third in the USATF Masters Steeplechase at 48.
Actually improved from early 50s through to about 56/57 - placed in USATF at 3000m s/c, 5000m and 3000m indoors, won some local 5ks - with better training (mainly including some longer runs up to 90 to 100 min, when I hadn't gone much over an hour before, other than training for a marathon).
At about 56 wrench an ankle. Got over that but at the time didn't realize that as it healed, I'd lost a lot of mobility. Ultimately it started an 'injury cascade' including a partial meniscus removal at 58.
Got back to some decent form quite quickly after that - ran 11:32 for 2 miles on road at 60 - but having still not discovered the ankle problem, had something blow up in fall of 2017. I don't know what it was, but caused huge swelling of foot, and inability to run for three months (for a long time I could only walk on the heel).
When I started back, there had been huge atrophy of the calf to the point where I couldn't do a single heel raise (couldn't get the heel off the ground) and it felt like I was running on a flat tyre on side.
Been battling imbalances and trying to get strength/mobility back into the calf since then. Finally, this year I've been able to do speedwork again, and at 65 have run 11:33 in the Hayward Field Masters Classic, and 19:49 for 5000m on road. Given the second and third in the USATF Masters on the track both ran 19:45.xx, I feel I'm probably close to back where I was, although the calf strength is still not 100%. Had I run the 19:49 on the track it's right up with my best times age graded. Potentially, I think there is some improvement left.
So, to answer the original question, I guess I really fell off the cliff with an injury at 58, but although it's taken 7 years, I think I've gradually clawed my way back up the cliff. In competitive terms, on National/World Rankings (which I was never anywhere near as an open runner) I'm back to where I was or a bit higher (although obviously the ranks are getting a bit thinner).
I still run almost every day and lately have been doing four track sessions a week (Easy Interval method). Haven't done strength training (although I have horses and get some work out with hay bales and bags of feed), but planning to this winter.
Probably helped that I have kept weight down. I'm relatively heavy at 5ft 8 1/2in, 147lbs, but that is lighter than when I ran my fastest 800/1500m as an open runner.
Other than that, probably having always maintained intensity, and never had long periods out (other than that 3 month spell mentioned above) has been positive.
My have also just been blessed on the genes front, in terms of athletic longevity, as I have a sister who is 60+ and has won World and National Championship events.