Why can't you drink until your 21?

Answer 1

Because it's the Law. They had to come up with some age to limit drinking. There's so much underage drinking, so many don't pay attention to the law.

It was originally 18, then in the 18th amendment alcohol sales were banned in the United States (prohibition). This was very unpopular and created the opportunity for a huge black market as well as organized crime. Alcohol was legalized again with the 21st amendment. They did not set the federal drinking age to 21 though. It is 18 but each state is required to enforce a 21 age limit if they want to receive federal funds for highways.