If Arnie can go on at 15 then why not Little Johnny who wants to fuck bitches in cancun or little Timmy who wants get all jacked so he can knock a nigga out and shit, yo.
Bottom line, most people using juice are fucking choads and shouldn't be using anything stronger than creatine but in an ideal world I would agree with you.. if someone knew all about their bodies response to training and diet before they popped then they could juice to get to their 'limit' and then come off and maintain that limit naturally, especially if they were young and weren't seriously shut down the way some older guy are post cycle.
if they'd been training a year or 10 it wouldn't matter. But in the real world those people do not exisit, hence the 3-5 years hard training rule before even touching juice/