Change the drinking age to 18

18 is the age of adulthood in the United States, and adults should have the right to make their own decisions about alcohol consumption. Turning 18 entails receiving the rights and responsibilities of adulthood to vote, smoke cigarettes, serve on juries, get married, sign contracts, be prosecuted as adults, and join the military - which includes risking one's life. So why can't 18 year olds by alcohol at 18?