People over the age of 18 should be allowed to consume alcohol at their own will, because if they can be drafted into the US Military and go overseas where the legal drinking ages are much lower, then we shouldn't be penalizing those who stay in the US in the reserves.
Furthermore, if any able-bodied men/women can be forced to enlist if we went into full-on war, then all individuals who are of the fighting age (18) should be given the ability to drink.
If one ABSOLUTELY has to regulate it, why not use the same methodology as with the driving priviliges? One can only buy/consume so much or they are over the legal limit for their age bracket?