If a show is for people 18 and older can you take a person that is younger but your 18 and older?

No

Nope

No you have to be 18 or older to get in. If it was an all ages show it would state that.

Yes...the 18+ restriction is merely for liquor sales during the show...

Yes you can, I think the above 18 is mostly for those who'd want to purchase alcohol. But if an 18+ yo is along with the underage it should work