Oooooh. Fantastic application. In some sense, there is indeed still a black market for alcohol, the market for underage consumption. First, let me preface what I am saying with this: minors should stay away from alcohol (I would even go as far to argue that even college students should stay away) as they are likely to not consume it in a responsible manner, which is not just bad stewardship in the secular sense, but sinful in the Biblical sense. That said, those that favor age limits on alcohol should consider that it does come with the costs of creating a black market: kids are consuming in unsafe environments, kids are fearful of asking for help if they consume alcohol and this fear can lead to them getting behind the wheel, etc. As I look at the costs of prohibiting minors from consuming alcohol, the costs seemingly don't outweigh the benefits (if there are any, minors that want to disobey their parents and consume do so whether it is illegal or not). I apply the same reasoning to all illicit substances.