Underage Drinking in the Old West

What constituted a man being old enough to belly up to the bar?  Usually it was up to the judgment of the proprietor or bartender.  It looks like Billy the Kid was hanging out in saloons by the time he was 18.  Billy Clanton was doing the same when he was a teenager.  They were accepted as adults at that point and so their presence was allowed.  I imagine that kids younger than that--those who were on their own, who worked as cowboys--could get in and be served. But that was at the discreti