If I am 14 years old, am I allowed to get a mark on my hand if I'm under 18 to go to an 18+ show? Or would I be able to go with a parent as long as they have their ID?

18+ is 18+ same goes for 21+ with very few exceptions. Call the venue ahead of time for full restrictions on attending.

Well we called the venue and they said no, I cannot go, but there's a page on Twitter called firstavenue and someone asked the same question, that page answered yes. The venue said no, but the organization said yes. It's very confusing.

Call them and ask. Doesn't hurt.

I'm pretty sure you'd have to go with a parent unless it's an all-ages show

Most venues will let you go to an 18+ show with your parent or guardian. Call the venue and ask first to make sure though, and bring nessasary paperwork that shows they are your legal guardian.

I would think it would have to be all ages

That's how it used to, but maybe this has changed in the last 10 years since I've run into this issue before.