The problem is that it's been 21 for so long that kids would go nuts if it was suddenly lowered to 18. Another problem is that younger people don't have the best judgement in the world, and drink too much. At least if it's illegal it's a little harder to obtain. Legal-age college kids on the whole drink far, far too much. Maybe that's because they have a feeling of "letting loose", or maybe it's because they are stupid, but it's probably both.
18 is a pretty good age, I think. 16 is too young, but at 18, it's good to put the trust in the person to maintain good judgement. Letting them drink is a good way to do that. I would lower it to 18 if it were up to me, but I won't deny that it would be ugly for a while until it became a "normal" part of society. There are other things to consider as well. Would the laws be adjusted? Now, if a teen gets caught drunk driving, they basically get a slap on the wrist if they're 18 or younger, and judges are much more lenient if you're younger than 21. If you're over 21, things get much harsher.
Also, Stapel says it's rare for 18 year olds to drive over there, but that is not the case here. I'd say about 100% of kids are driving by the time they are 18 where I live. In the big cities I imagine it's much less, but still quite a majority, I'd bet.
The bottom line is that when someone turns 18, they are expected to be an adult. The law should treat them as one, and that includes the benefits as well as the punishments.
Either you want to have less drunk driving related accidents or you don't.
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Ok, so i forgot two letters in my last post.. big deal 
