Sorry, but I can't buy into the "teach them to drink responsibly" theory.
First, there's the health problem: Their bodies aren't ready to handle alcohol. 2 beers to an adult liver and 2 beers to an 18-year old liver are not the same thing. If my father had not been allowed to drink in his teen years, I believe he would have lived longer. He may have become an alcoholic anyway, but I don't think his liver would have shut down before his 40th birthday.
Then there's the permission problem: Kids who are allowed to drink at home almost always DO drink other places too -- they see it as "mom and dad are okay with this", so I'll go ahead and do it. Recently I went on a trip with a large group of friends, some of whom allowed their teens to drink ONLY IN THE ROOM WITH THEM; those teens didn't follow their parents rule -- they drank everywhere. Once a privledge is granted, it's hard to take it back. On the positive side, I think most kids HAVE finally understood the "don't drink and drive" thing; it doesn't stop them from drinking, but they do plan their rides, which is a step in the right direction.
Then there's the legal problem: It is illegal for kids to drink under age 21. Whether you agree with this personally or not, it is the law. If you allow your kids to break this law, you're basically telling them that it's okay to break other "unimportant" laws too.
Perhaps more importantly, I don't believe they're automatically going to drink. I didn't drink during my teen years -- not at all, and I wasn't alone. I've discussed this with my brothers, and they say the same thing. They just didn't. Part of this may be due to the example we had in our father, but I think it has more to do with the fact that our mother and our stepfather made their expectations clear: drinking is for 21 years old and older. Of course, they didn't drink on a regular basis -- they didn't see alcohol as a necessity for a good time, so they set that example before us. I know quite a few teens today, and MANY of them tell me that they and their friends DON'T drink. So the "everyone's doing it" theory is just bull.
Here's what I think the real problem is: Americans have developed the idea that one MUST drink to have a good time; the more you drink, the more fun you have. It's an unhealthy attitude. If Americans were good at having one drink with dinner, it wouldn't be so bad -- but our society tells us to have one before dinner, two with dinner, and a couple more in the evening -- otherwise, it's not a party!
You don't have to agree with me, but please consider my reasoning. I believe it's valid.