Can someone under the age of 18 get into a show if they have an adult with them?

Yes.. to an all ages show with adult

Unless specifically stated as an all-ages show, I believe you have to be 18 with an I.D to get in. But the bar itself doesn't card to get in when there's no show: just if you try to buy or drink alcohol.

What they said ^^^totally depends on the show, check first