America is one of the few countries in the world that restricts alcohol until you are 21 years old. By the time you are 18, you can join the military and kill for your country, but you can't drink a beer.
There's no denying underage drinking, specifically underage binge drinking is a huge problem in the USA. I believe that this binge drinking and sheer irresponsibility comes from a lack of proper alcohol education, coupled with a very altered mindset that can only be described as kids drinking so much because alcohol is illegal.
With proper alcohol education, including teaching youth to learn their alcohol limits, and portraying alcohol as a drink rather than an evil mystery drink, we can lower underage drinking and have more responsible drinkers, allowing the drinking age to be lowered.