I actually think that the drinking age should be 19... By that point you are usually out of high school and are living your adult life (college, working, in the military, etc.). By the same token, I believe the age for smoking, voting, *bleep*, enter into contracts, serve in the military, etc. should also be raised to 19. The major issue I have is that I think if you are going to have an age of majority, then it should be the same for everything. But one of the major reasons why I've heard they don't want to lower the drinking age to 18 is that at that age, there are more still in high school and have younger friends, so the younger ages will have easier illicit access; making the age 21 still allows their younger peers illicit access, but people are more comfortable for an 18-year-old college student drinking than a 15-year-old high school student drinking. Thus lowering the age to 19 will still keep the easy illicit access out of the high schools, and also allow adults in every other aspect of life the opportunity to experience alcohol.
I also agree that making something taboo actually has the opposite affect of stopping underage drinking. The adolescence period of a person's life is a time when the person comes into their own. They examine the values and reasonings they were raised with, and decide whether they still accept these. They tend to explore boundaries to see if the boundary is truly a hazard to their life and/or lifestyle. Thus making something like alcohol taboo until a later age is telling the person that on the one hand, alcohol is ok (adults can drink), but on the other hand, an artificial boundary is imposed preventing the person from making that decision in their own life. I think that is why European nations, and other cultures where there is no strict drinking age, sees less binge drinking and other dangerous behaviors involving alcohol. If kids grew up where they were allowed to drink, say a glass of wine at meals or sharing a beer with dad while watching football, they would have a more informed idea about what alcohol does to the body, and would be less inclined to sneak out of the sight and control of responsible adults the exploration of this avenue in life.