RB, the topic here was about Rebbeim not parents if you specifically want my opinion about parents who allow their underage children to drink ask me again and I will answer.
Arc, in E”Y where the legal age is 18, then the talmid is in charge of the choice to accept or decline and the Rebbe should respect that decision and in no way should he be insulted if the Talmid declines the offer. A Talmid should still be respectful to the opinions of his parents as well as to the seriousness of the issue and its consequences. Drinking alcohol is a serious issue as we have all learned all too well. Starting to drink at an early age is of no benefit to anyone.