I think most posters are in agreement that giving alcohol to a minor who is not ones own is off the charts. DH and I often host open houses where sometimes 70 odd people come. Some have teen children, and we normally just set out a cooler of beer/wine, and another with non-alcoholic drinks.
I am very aware of peoples' different rules for their kids, and would never dream of offering a drink to a minor. I would let that be initiated by the parent. AND--you bet your bippy that if i saw young "Tzyphaknee" walking around with a wine cooler I would approach the parent and ask if that was ok. (Maybe parents offered, maybe she was sneaking it)
If the parent says it is ok, that is their business, but if not, I would explain to the teen that it was not ok, and I would suggest a lovely iced tea.
Even in my own home, I respect the parents judgement regarding the child.
A friend of mine has a son, who is 16, that wants to invite 2-3 friends over, and see what being drunk is like. She asked me for advice, and feels that if the kids are in the home, not driving, etc. that they will be safe, and under a (not too intrusive) eye. I told her to call the parents of his friends that he would like to invite, and ask how they felt. Involve them in the discussion, and let them know that there will be alcohol. I also warned her that she is taking on a huge liability by serving alcohol to minors in her home. I suggested that maybe she and the other moms may wish to have a party of their own in the kitchen, while the boys "do their thing" in the den.
Another karma moment for me happened this New Years eve. 16 year old girl, (the hosts niece-in-law) her dad said it was ok for her to have a glass of champagne and one drink/beer/glass of wine. He told her that two was her limit throughout the night. The host and I were sitting near the bar, and did a pretty good job of enforcing that she would be drinking diet cokes after her second. She batted her eyes at the hosts brother who snuck her a third drink, without anyones knowledge. The hosts brother was also the one covered in throwup shortly after she finished the drink. I think that may teach him to respect the wishes of others. Hee hee