The creeping infantilization of American adults continues.
That assumes that 18 to 20 year olds should be considered full adults. Personally, I don't think they should be, tho to be honest, I didn't think so when I was that age.
They either are full adults who can vote, be drafted into the army, enter into contracts and be tried as adults or they are children. There should be no such thing as a partial adult who can vote and be drafted but can't drink or smoke. The age of majority should be uniform.
Honestly, raising the military service and voting ages to 21 might make a lot of sense. The age should be uniform, though.
Or if we want to Biblical
age 20, tho the fact it's a nice round number is a better reason to choose 20 than because it's used in the Bible. That said, I don't see the problem with giving older adolescents some but not all of the rights and responsibilities of adults.