It is a dangerous drug that directly affects the brain, and a not unreasonable compromise with Prohibition (which was famously repealed and which sets the regulation to the states). The Federal government pressures the states to set it at (a minimum) 21, and ties it to the Federal Highway funding to each state. 18 is the age of consent, to sign contracts and checks... Alcohol inherently negates judgement. For those who think 18 is too young for consent, it at least removes from 18 year olds a handicap to their thinking, giving them more time to develop better self control. Limiting the ease for getting drunk in public for a few years is unlikely to cause them great harm. We limit Political offices based on age, because of their presumed capability of judgement.

I do, though, wish we could be consistent in what age we think we're old enough to make responsible adult decisions. Drinking, voting, enlisting - they all require a level of maturity because they have consequences. Whether 18 or 21 (or 16 or 25, for that matter), let's just be consistent.