I agree with dealing with the real problem which is disruptive children at meetings and workshops rather than skirting it and setting an age limit. Younger women are welcome but leave the kids at home. Some people have no boundaries and you have to lay out the rules very clearly and then STICK WITH THEM. Once you start saying "It's okay for so-and-so because she has a good reason" the rules go right out the door. It can be hard to be the bad guy and enforce the rules but it's got to be done.
If people don't want to follow the rules, you may have to have announcements about "no young children at meetings and workshops" for a couple months and then start turning those away with a kindly "I'm really sorry but we have announced it at the last few meetings that young children will not be allowed at meetings and workshops so we can't let you in". It won't be popular, but there it is.
I would actually lower the age limit from 18 to maybe 13 or 14 and then if those kids are disruptive deal with it on and one by one basis. There are lots of teenagers that could come to a meeting and enjoy it and maybe learn something. It might spark some interest in quilting with them.
Edit: I just thought of something. With as many grandparents as are raising the grandkids now, I'm not sure the age limit would work anyway.