The fed says you have to have guide lines, Here is some guidance, the age limit can not be lower than 18, we don't care how HIGH you put it.
I working in a bar in 1978, 79, 80, 81 and 82 during the transition from 18 back to 21. It WAS left to the states as too what they chose to raise the age limit. It was a real pain to try and keep track of those that were on the cusp, were grandfathered in because they had already passed the limit but were too young again the following year. Michigan raised the age from 18-21 over a three year period and it was a total clusterf**k...
But, it was left to the states, just like other so called rights that are NOT enumerated in our Consitution! the Feds control the booze commerce and transportation issues but it's left to the states how they administer them. For example, MI never allowed a bar or nightclub to serve alcohol that was not purchased from a licensed distributor. You could not drive down the road to the local packy and serve the booze you bought there. Completely illegal.