Wedding guest list and invites

Your best bet is to invite in circles. Setting an age limit is going to make things complicated and probably bring a lot of drama. What will you do for a family that has an 18 year old child and a 16 year old? Invite one sibling and not the other? It would be best to just not invite either of them. If you don't want children, invite the parents, but none of their kids.

You could also check with your caterer and see if they offered discounted kids meals. Ours had steep discounts for kids under 10.
But if you're dead set on no kids only send the invite addressed to the parents, put on the RSVP card "we have reserved 2 seats for you" to make it clear the whole clan isnt invited.

We put on the info insert in the invites thatbit was an adult's only affair and to leave the littles alone. I also addressed the invites to only the people invited and put how many seats we had available for them to use. That way they dont think "oh we've got two seats, mom isn't able to make it so we'll take little Johnny instead." We also circulated it by word or mouth that there will be no one under the age of 18 there. And in my family all of our cousins are either way older or still under this marker so it isn't an awkward cut off for us.

I out it on my invites, and envelopes.

On the envelopes I addressed the save the dates and invitations to Mr. & Mrs. Smith instead of The Smith Family.

On the invitations I wrote on the bottom *Adults affair only* and then on the RSVP part I put

" We have __ seats reserved in your honor" and are going to fill in the __ with a number.

All of our friends and family have kids under the age of 12, so that made the "no kids rule" a lot easier. That alone saved us from 30 additional invites!

We addressed the invites to only the adults, stated how many seats will be reserved for them, put it on the website, and by word of mouth. So far no complaints.