In that case they should also do away with prohibiting them from buying alcohol. 18-20 year olds are either adults or minors. Not 1 part of one group one time and another the next. If they can vote, be drafted to fight and die for the country (the ultimate mandate) and be tried and sentenced to prison, they are adults. Neither the 18tth or 21 amendments mention age.
As someone who bounced when the drinking age was 18, I have observed 18 year olds who behaved in bars as well as 40 year olds who didn't.