I think a lot of it is just experimentation. Taking drugs, having sex, partying, etc.. are all ways to experiment with your boundaries and discover who you are as a person. I don't think there is anything wrong with that as long as it is done in a safe way and they are aware of the risks involved. I think the more you prohibit a behavior the more likely a person is to do that behavior. In countries where drinking alcohol isn't a big deal you don't get teens trying to binge drink themselves to death or becoming habitual drinkers. It is a parents job to inform but ultimately the teen will make the decision. Hopefully it won't kill them and they will learn valuable lessons from it.
I drank before I was legal but I did it with close friends in their homes. I wasn't out driving or in a sleazy neighborhood. I had a place to sleep and was around people I trusted. I have only drank on occasion a few times a year during the holidays or get-togethers. It is a nice addition but not necessary to have a good time.