It's a tired, overused argument, but there much rationality in it and I think it should be looked at with some level of scrutiny. As others have stated, at the age of 18 we are old enough to go to war and kill other human beings; we can smoke, which is just as dangerous as alcohol in the long run. We can vote, which may change the fabric of our own society, a massive duty that every American should take seriously. While the brain may not be fully developed until the age of 25, alcohol in moderation will not be nearly as dangerous for an 18-year-old as some might think. For example, some wines produce positive effects when drunk in moderation because of their antioxidants.
While not all alcoholic beverages provide positive effects, it should be noted that there is a heavy psychological aspect to this. Young people often see forbidden acts as personal challenges; tell a teenager not to drink, smoke, or have sex, and they're more inclined to do so. It is a taboo that gives the person a rush. It's something illegal, something "cool". Yet it is only "cool" because it is seen as something that goes against the mainstream opinion. If drinking alcohol were not as serious of an "offense" as it is today, I would expect that drinking alcohol would become a more normal activity that would become less "thrilling", something that would not require that the person become drunk to enjoy the beverage. When we stigmatize a product or an act, it only makes it more appealing.