I don't know if my own experience offers any perspective on this. I was what I'd call sub-competitive as an early 20-something: not good enough to make college XC, certainly. Best times by 25 were 5:02/17:27/35:49/ and a marathon in 2:53:30.
At 26 I quit racing cold turkey, and for the next 19 years I basically just jogged 3-4 times a week. Never ran more than an hour. Never ran a formal workout.
At 45, in 2003, I raced again for the first time in 19 years (a 10K more than 10 minutes slower than previous 10K), found this website, caught the bug, and began to train.
My times got faster from age 45 through 51. I lowered that 10K from 47:25 to 40:42. Best 5K was 19:28. HM in 1:30:11. BQ at 51 was 3:30:59.
In age-graded terms, the 5K/10K times were faster than my mid-20s times.
Injuries began soon after that, but I had six great years in which the next PR was always within reach; I felt fresh, fit, powerful, and came back pretty quickly from injuries.
I'm quite sure I wouldn't have had the same experience of my late 40s and early 50s if I'd been training straight through that 19-year period.
I suspect there are others in this age range who had similar latency periods, maybe gained some weight, then got hungry to run and train. Certainly my mini-renaissance was helped by this website and by my nonstop consumption of books on how to train.
For what it's worth, the ONE book that put me over the top and that I've just gone back to is Brad Hudson's and Matt Fitzgerald's RUN FASTER:
https://www.amazon.com/Run-Faster-5K-Marathon-Coach/dp/0767928229/ref=sr_1_1?ie=UTF8&qid=1493293598&sr=8-1&keywords=brad+hudson+training