They can but their competitors will reap the benefits....if a Stae edicts it,, then we’ll have the same situation we did when I was growing up and PA had a 21 year drinking age and NY had a 18 year age - lots of 18-20 year old boys and girls would cross state lines to be able to indulge....which put them in more danger than if they could have gotten it at home.
I'm of your vintage and can remember the same situation in the southern tier of PA with MD having the lower age.
Then MADD got involved, called them "blood borders" and the next thing you know Congress is extorting Federal highway funding to force all states to go to 21.