History Speaks for Itself: Prohibition Doesn't Work
We’ve been there before. During the 1920’s, America undertook the “noble experiment.” The outcome then sounds a lot like the present. Alcohol consumption declined, but reckless, excessive drinking went up. Criminal behavior increased, and respect for law diminished. The speakeasy and the bootlegger became familiar terms as drinking simply moved behind closed doors and underground. The cost of enforcement skyrocketed while the rate of enforcement remained frustratingly low. By the late 1920s a growing body of responsible citizens decided to attempt the impossible: repeal of a constitutional amendment. In February 1933 Congress passed the language, and by December 5 the requisite number of states, by overwhelming margins, had ratified the 21st amendment. How many times must we relearn the harsh yet predictable lessons of Prohibition?

I am baffled by this "Prohibition Doesn't Work" argument, which frankly strikes me as unworthy of its presenters. It simply doesn't follow that because a sweeping prohibition of all alcohol for all ages, decades ago, didn't work, that it is therefore a bad idea to prohibit alcohol for 18-20 years old. If the generalization that prohibition doesn't work holds, then why prohibit alcohol for 14 year olds? The issue is not whether it makes sense for society to tell a 40 year old that he or she can't have a drink after work. The issue is whether 18 to 20 year olds, as a group, are different from older adults in that they are at more risk for drinking-related problems. That issue is fair and raises empirical questions. "Prohibition Doesn't Work" is just rhetoric.
From my limited understanding the prohibition cure was worse than the disease. Congress, instead of trying to remove the side-effects decided that it was better/easier just to repeal prohibition. The current limited prohibition as currently written is "wrong" only in that it contradicts established protocol related to private goods regulation for individual use. Additionally, it punishes people for partaking in an activity that many people, of all ages, can and do perform responsibly for entertainment and relaxation.
Given our existing framework of laws and regulations as a basis I do not see enough about alcohol that is different from existing drugs for it to warrant special treatment. Current drugs are either fully prohibited or restricted to use by adults, the age of which - while undefined legally - is effectively 18 since individuals are automatically emancipated at 18 if they so choose (or their parents kick them out).
One being at higher risk for drinking-related problems is not a basis for banning that group of people. Women are at higher risk for brain damage from alcohol consumption, but it is not logical to ban alcoholic consumption for women. Likewise for fully aware and consenting young adults.
Also a quick side-point which questions that 18-20 year olds are even at greater risk, refer here. You can find the studies cited on the bottom:
http://www2.potsdam.edu/hansondj/InTheNews/UnderageDrinking/20060830151926.html
I had a professor, Dr. Dwight Compton who often said, "An ounce of history is worth a pound of reasoning." The history of the prohibition movement was that alcohol was then a right and alcoholism rates were at a historical high. With no known treatment except for a deep religious experience it seemed like the best choice for all. They had the problem one/half right, this issue was alcohol, but what about the role of the individual and the environment where they lived? Prohibition did work for a period of time. But it was a prevention model that would ultimately be doomed. Delaying the age of use should not be linked to the Prhibition movement. We must reduce alcohol related problems by finding the right prevention model that works in the 21st Century.