in the US, its 21. probably because people who are in charge of making those decisions are hoping that with a more mature age, that people will make better decisions with alcohol use. however, in several countries in Europe, children as young as 6 are allowed to drink alcohol, and they have a lesser percentage of problem drinkers/alcoholics than the US!
18. Since the legal age was raised to 21 in America, cases of drinking and driving have risen. No one really has a reason, but I could suggest that kids are going to drink at 18 no matter what you do. Instead of teaching them to drink responsibly, you are making a situation where they are already breaking the law drinking at 18, and perhaps many just say, "I'm already breaking the law - whats breaking one more law?" and going ahead and drinking and driving. (Just a suggestion - like I said - no one really knows why this is happening).
In addition, Canadian provinces where the legal age is 18, there are less drinking and driving accidents than provinces where the age is 19. Numbers don't lie, don't fight the situation, work with it!