I'm the youngest in my family so the "no kids" thing is pretty straightforward for my side (there's no way my cousins would conceive of bringing their 1- and 2-year-olds), but on FI's side, he's in the middle of the pack. Specifically, he has one cousin who will be 18 when we get married and that cousin's sibling who is 14. We're having alcohol at the wedding and so we thought it would be a simple cutoff to not have guests under 21, but his 18-year-old cousin isn't exactly a kid (though I would expect etiquette would dictate we can't invite one and not the other). We're both mid-20s, and there won't be anyone younger than 24 there aside from these cousins (if invited). What does etiquette say here? What's the defensible age cutoff?