I'd strongly question that assumption, from personal experience with proper training and feeding, young lads are still growing up to about 17 & 18 some even up to 19. I've seen Junior Entries in the Military who've joined at 16 & 16+ as weedy individuals finish their training 6 months later as rivals for Capt Hurricane and then, when they've gone out into the big wide world of the British Armed Forces continue to 'grow' until they're Charles Atlas lookalikes!Apparently, according to the nhs, the growth of most boys ends at around 16 years of age, although they might gain an inch in height within a year or so. Muscles keep developing until the age of 40. I was a bit surprised by both of these facts.
You only have to look at young Hartridge to see how that also happens in football, at the end of one season he was of average build, then the next he wouldn't have looked out of place in a No 4 shirt at Sandy Park.