Why should a server be cautious even when a group includes both adults and possible minors?

A server should be cautious with mixed-age groups because an adult of legal age might purchase alcohol and then pass it to an underage member of the group — a practice that is illegal and that the server can be held responsible for if they reasonably should have known. Watching for this kind of 'second-party' or 'straw' purchase, and refusing service when it appears alcohol is being obtained for a minor, is part of responsible service. The seller's duty does not end once an of-age buyer is identified.