I was one of the last of my friends. They would buy for me under conditions, and I do the same for my 19 year old brother.
The condition: don't drive; don't start fights; and if you toss your lunch, you're cut off. I don't think there's anything wrong with 18+ year olds drinking, as long as they do so responsibly. My parents let me drink around them starting at 17, and I believe it allowed me to learn how to use alcohol responsibly, so that I wasn't cut loose at college with no prior experience.