Do they thoroughly id check?

Is it 21+ only

I believe so.... When I was there, they were ID checking in the line outside the theatre and putting bands (tightly) on the wrists of the people over 21. I don't know how well it was enforced inside the theatre but if you're that desperate to buy overpriced booze while it's illegal, you may want to review your priorities. :-)

Depends on the event

Yes