In the United States, citizens are considered to be an adult when they are 18 years of age. This is also the time when they are deemed responsible enough to be exposed to tobacco purchase, join the armed forces, and also help decide who is put in charge of the entire nation. It is also mandatory for males to sign up for the selective service.
However, in the eyes of the American government, 18 year old men and women are not responsible enough to be put in charge of alcohol for their own consumption. We believe that if someone is responsible, mature, and trusted enough to be able to decide out country's leader as well as lay their life down for millions of people that they don't even know, then they are entitled to be able to purchase as well as consume alcoholic beverages.