I have noticed, from anecdotes, that child-free weddings end up being child-free with exceptions. Those exceptions then make it a bit of an insult to the parents who put their kids in someone else's care to go, when the kids in attendance were just as 'child-like' as the ones left out!
I'm not down with child-free weddings, either - if a wedding isn't a family event, I can't figure out what is!
Plus, the age-cut off, upper and lower is too vague. My mum didn't take us to a family friend's wedding when I was about 12 or 13 and my sister was 10...not sure how we were going to impede on the day...and of course, my mum reported loads of kids there anyway, despite the invite specification
We weren't sure if DS was invited to my cousin's wedding, but figured he was cos they knew we were bringing him all the way from Oz (wedding in Ireland)! Turns out he got gastro and gave it to me that night, so he was no problem with exuberance Breastfed 19 month old, who would have ordinarily been tearing around the hotel, who was coming regardless of no-child policy (wedding was in her fiance's county, where I have absolutely no family and didn't want to leave DS with perfect strangers!).

I think that if guests want to have a child-free time, they'll organise child-free themselves. If they would rather have kids there, it's not like the bride has to hold onto them and get her dress ruined. Personally, I think inclusion of kids makes a wedding more fun and joy-filled