I believe that once an American obtains the opportunity and responsibility to vote for their country and die for their country, they should be able to consume alcohol in their country.
Separately, in combination with the Federal Aid Highway Act, the National Minimum Drinking Age Act of 1984 basically made states choose between raising their alcohol consumption age to 21 or losing 10% of federal highway construction funds. While this did not explicitly violate the 10th Amendment, it does seem to create an unfair decision.