Why dont I as a 19 year old adult have the right to drink a beer....?

This is unfair. How come from the age of 18, your legally an adult, your old enough for your parents to no longer be responsible, your old enough to be a star, your old enough go to jail and receive the capitol punishment, and your old enough to be join the military but why am I still not old enough to buy a beer? This is discrimination. Why am I and people 18, 19 and 20 being treated like criminals and not citizens? In the US, if at the age of 18 you are considered an "adult" and are solely responsible for ALL of your actions. Instead of trying to ban a small group of adults, why dont they just make the drunk driving laws much more strict so people will think twice about it? This crazy why some people who are responsible have to have everything ruined by other idiots. What about the alcoholics way over 21 who drive drunk? Why is there no law to stopping them from that right? So they are irresponsible to drink, but responsible to vote and enlist in the military. How do you justify that?