Why should teens, specifically, stay away from AAS? The first most obvious answer is because they are already on steroids, naturally. Teen males have so much testosterone going their bodies that adding synthetic testosterone will just shut them down, being counter-productive. The more serious reason teens should stay away from AAS is because they will stunt their growth. "Finally, some bodybuilders felt teenagers should avoid steroids given the risk of premature physeal closure and the stunting of growth (cf. American Academy of Pediatrics 1997: 905-6)" (Monaghan, 39).
Monaghan, L. (2001). Bodybuilding, Drugs and Risk. New Fetter Lane, London: Routledge.