When younger athletes max out their training capacity, they either come close to pro level (where any doping would be out of scope for the question of the perceived age correlation in recreational doping) or they lose interest. They lose interest either completely, or at least enough to be fully aware that more training could make them so much better. It's easy to resist doping if you know that the amount of training is your main bottleneck.

Age groupers are much more persistent, they have their hobby permanently dialed in, they wouldn't even know where else to search for a sense of achievement. They are used to getting faster from a combination of better technology and better training, and to some of them doping might seem almost like a natural extension of carbon and training plans when those are maxed out. Fortunately, the deep end of both the training and the carbon is so far out that I am confident that it's still only a small minority who falls for the temptation (the very few testing that has happened on the amateur level concentrated on the top results, if doping works, that's where you will find the dopers)

(source: getting more carbon and more training as I get older)