The problem isn't just that 1 rule, it's the whole system around it.
First of all, if people want to do something they're going to do it, legal or not.
Second, the real problem isn't alcohol, it's the people drinking it. for example when i drink first make sure i can spend the night at that friends house (i don't drink at bars) and even drunk off my ass i know enough not to drive.
Third, to everyone saying that people at __ age aren't mature enough to drink, that is true for most people of any age.