I was at an 18+ Opeth show at the House of Blues in New Orleans, and they didn't check IDs at the door. I had two younger friends that really wanted to go but couldn't because didn't wanna waste money on a ticket that could end up being useless. Just hope they don't check, and if they do, try telling them that you were at the last 18+ show they had (you'll have to check their website to see what band it was), and try to get some of your fellow metalheads around you to vouch for that, saying you came with them.
I dunno, just a thought.