I've had experience with drinking but I'm not really going to go into that.
For the most part, I believe most states in the US have a law (well at least where I live) that allows under 21/above 15 to consume alcohol as long as it's on their own property and with parental consent/presence (forgot which). So, in most cases it would be up to the parents and likely only during special occasions like maybe new years or christmas or something. To me that law is perfect. Parents should really teach kids to drink responsibility rather then not at all and the law i mentioned is almost a good way to do this.
Don't get me wrong, it's fun to get drunk off your ass but really bad to make a habit out of it obviously.