I am an underage college student at one of the most notorious party schools in the country, and I see first hand why a 21 drinking age is a problem. Everyone in college drinks, regardless of their age, and that isn't going to change. The problem is that someone underage really only has access to alcohol at large frat/house parties. Since they cannot go buy some alcohol or go to a bar, this is the only option many underclassmen have. At these parties, it is not only encouraged, but basically mandatory to get completely trashed. There is no moderation. If you are of age, you can go to a bar and have a couple of drinks to relax. There is no pressure to get drunk. But, when alcohol is a forbidden fruit, people can't help but over-indulge.

Also, at 18 one can: get married, smoke, vote, fight(and die) in the military, and even bartend! So someone underage can make you a drink, but can't taste it themselves. This is outrageous.