That answers the legal question of why the drinking age is 21, but what was the underlying logic of the original policy?
Did lawmakers just pick 21 out of a hat because they wanted college seniors to learn the nuances of bar culture before graduation? Not quite. The concept that a person becomes a full adult at age 21 dates back centuries in English common law; 21 was the age at which a person could, among other things, vote and become a knight. Since a person was an official adult at age 21, it seemed to make sense that they could drink then, too.