IMHO, kids are naturally drawn to the forbidden. I was drinking in the streets before I was 13 and knew I didn't want my kids behaving that same way.
I've allowed them all to taste wine and beer. I served them a half-glass of champagne for New Years'. (One liked it, the other spit it out
) I did the same with my older daughter (she's 24) successfully.
That said, I would never provide alcohol for any of their friends. My home and supervision for my own kids is one thing, but I would not feel comfortable with them drinking anything anywhere else and would have to make that same assumption on behalf of other parents.