Interesting OP & thread.
I remember reading the red youth book (sorry can't remember its full title - it predated the blue Questions Young People Ask book) and it had a chapter on youths coming of age and being able to drink alcohol if they choose.
From what I remember, the advice given by the writer was that when a youth reaches legal age, he or she may have one drink. Not two or three, or more if they weren't driving - just one drink.
I get the feeling that the GB would ban alcohol for JWs if it wasn't mentioned in the bible ... kinda like they do with masturbation.