I think that when it comes to alcohol, it should be eighteen. The issue however is not how a body is physically unable to deal with alcohol, but I agree with others before that it is primarily a problem with education.
First, I'm not saying that alcohol has a negative effect on minors, it does. But it really only has negative effects in excess. Its exactly the same as those who are older, where alcohol is excess is harmful. The only difference is how much is excess. I'm not saying give a full beer to a minor. But having a little bit isn't bad.
I don't think that schools can effectively teach this sort of thing, because schools cannot teach responsibility. They can tell students to just not drink until they turn 18/21, but after that its fine. They can tell kids not to drink in excess, but they cannot define the limits of what is or is not excess. I think that when it comes to teaching about alcohol, it has to be parents who teach kids to deal with it. My own parents were pretty open with alcohol. My father always said he would rather me be allowed to drink and do stupid stuff at home than out with my friends. So around 15-16 my dad would let me have a small glass of wine on family Friday night dinners. At 19 he let me have a beer after we built a fire pit and patio in our backyard. Even before 21, I understood how to drink without going overboard. I just turned 21 about a month ago, and on my birthday I had a single beer. One. Since then, the most I have drank was actually last night after a very stressful day where I had a single beer and a small glass of whiskey. If parents were actually willing to teach their kids to drink responsibly, perhaps the whole getting drunk problem would go down.
A lot of the problem I think comes when those who are underage or just turn 21 and get their hands on alcohol don't understand the purpose of alcohol. I believe that getting drunk is not the purpose of alcohol, its a side effect. Alcohol is enjoyable to drink and makes me feel good, but its not something that you purely use to get drunk, which is perhaps what a lot of people actually want to do with alcohol. Its kind of a mindset problem.
Another reason I actually think the drinking age should be lower is because 18 year old kids can't usually afford a whole lot of alcohol. At 21, I'm making much more than I was at 18, and could buy much more alcohol. At 18, I might have been able to afford a six pack per week. Maybe. If we lower the drinking age, perhaps younger drinkers can be introduced to alcohol while not being able to afford heavy drinking.
In general, I don't care about the drinking age. It can stay, or it can get lowered, but the issue is that people need to be taught to drink alcohol responsibly, and schools are terrible at doing that. I think its something that parents have to do, actually do their job as parents and teach their kids.