As a rule of thumb and there could be exceptions, 18, in a good number of jurisdictions, yes, Alabama had dry counties where no one can drink but 18 in general, 18, old enough to drink, old enough to vote, old enough to enlist.
You can have exceptions, but this looks like a behavior that Moore took part in perhaps repeatedly. Then, we can say, well, it was still legal, etc. etc. We've been through some of these arguments already.