Yep, lots of great ideas- but restricting it to 18+ is NOT one of them.
First off, I've never been in a con that allows alcohol, and it's super easy to get people to line up at the door and check their water bottles, bags, badges, id whatever before they're allowed in. It may have to start a line a bit before the dance officially starts but that's the price to pay for safety. I'm sure more people would rather wait a short bit rather than not be able to go at all because they're 16, 17, or they're a few months short of being 18.
As for security, after the door check we've got the staff who are in charge of keeping a watch over the place. They can't be everywhere at once, but they'll be there. And it's pretty easy to spot anyone who's acting up. Don't forget the people who are there to enjoy themselves- someone usually has the sense to report to the staff.
Also, if it's a later time like 9 or 10, not gonna see a lot of kids there anyway, if at all.