The US shouldn't have a minimum age limit on alcohol. It's a victimless crime law. Ugh.
It's only legitimate to stop people drinking when there's some danger involved, e.g. drinking with driving. One can argue for other cases like drinking heavily with a "mean drunk" personality type, or drinking heavily without the experience/knowledge to be safe in that situation. Drinking with being aged 20 isn't inherently dangerous. And it's a bad (both inaccurate and ageist) proxy for the other things that actually matter.