I'm talking about serious amateur runners/hobby joggers or even once decent competitive runners. Lets assume they have carried on as dedicated hobby joggers into their 40's, 50's, 60's, at least 20 miles a week, and no serious injuries/health concerns (other than general aging) that would set them back years.
At what age does it become permissible to no longer be able to go sub 20 in the 5K while still running regularly?
I'm guessing around 52/53 y.o.?