I don't drink alcohol, and don't ever intent to. But I do think that the age needs to be lowered. I mean, it's not even difficult for those that are under 21 to get a hold of it anyways. If they want to drink, they're probably going to drink.
I'm certain the biggest issue regarding alcohol consumption is the lack of education regarding it. I don't think people comprehend to what extent it affects you, and that's why they drink irresponsibly. I'm not just talking about the short term, either. Most people I've met don't even seem to comprehend the long term effects it can have on your body. Even though, at least to me, it should be common sense that something that obviously alters the way you think would be bad for you. A couple drinks here and there aren't so bad, but I know there are a lot of people who get drunk every night. (I work with a few, actually)
Besides, most people don't change in maturity between 18 and 21. I think the biggest problem between those ages is peer pressure, which is going to be a problem no matter what age you can buy alcohol.