I'm not going to patronise milkybarkid. I remember 10 years ago, going to parties when I was 17 and doing exactly the same thing. I wouldn't suggest going over the 21 units every week, but every now and again is not going to do much harm. The much bigger problem is antisocial behaviour brought about through drinking, and I'm talking about kids throwing up in the street, ******* against buildings, or acting in a threatening manner. As long as they're behaving themsleves, and acting with a respect to the rest of society, I'm not against a few (older) teens getting drunk at a partyat a weekend. Just understand your limits, know when to stop, and make sure your friends know the same.
I draw the line at young teenagers 15 and below though, who on the whole are not well equipped to make the correct decisions about this sort of thing.