With the National Minimum Drinking Age Act of 1984 states were forced to raise their drinking ages to 21 or face a 10% decrease in federal highway funding.
The U.S. has the highest drinking age of all developed countries and appears to have the most problems with underage drinking.
We are depriving adults, ages 18-20, their right to consume alcohol; when they can vote, serve on juries, eligible for military draft and service, etc.