They also run the risk of messing up their endocrine system and will also not have reached a natural plateau.
They can choose to do it even earlier, its not like at 18 that it becomes legal. The issue is that they run the risk of messing themselves up by following only the advice they want to hear and come running once they have issues.
I've seen many 18 year olds on here who say they understand the potential consequences of their actions... But until it actually happens they don't realise what they've done.