In our case, there were very few people under 18 who would likely be considered (extended family that we rarely, if ever, saw). We were relatively young and our DPs (both sides) were very controlling of it generally including that they wanted significant numbers of their friends to attend. As numbers in our venue were limited, we had a child free policy (it was a family wedding with those DP friends who were insisted on - neither of us had any friends invited....if that helps indicate the levels of control and angst we dealt with).
My DCousins were generally mid teens to early 20s but lived hours away and overseas. In my family, weddings were always seen as an adult only affair - I got the impression at 1 point that my DGPs would have been happier to exclude at least some of my own DSiblings as they ranged from 15-24.
All siblings of B&G attended and 1 babe in arms. Otherwise, there was a 9 year old who unexpectedly arrived but whose parents assumed the invite, despite being worded only to "Mr and Mrs X", had included the DC as "everyone knows we never leave her behind".
If I had a choice of including even more "family" whom I never saw as well as all the extended DCousins etc (some of whom I met for the first time ever at the church!), or actually inviting a few of my own friends, I would have chosen my friends.
Ours wasn't an entirely traditional event though as while we had the proper meal etc, it was not a big function room but a series of dining rooms in an older country house, and all retired to the drawing room for a singsong or conservatory to chat in more peace after that. No dancing and no chance for running around on a cold, very wet day in middle of winter.