There is no checking of ID's, so it is your own responsibility to make sure that your hook-up is not underage. What "age" constitutes "underage" in this particular context is unclear -- though 18 should probably be safe.

PS: I mean "safe" from getting in trouble for having sex with a minor. Getting in trouble for having sex in public is a different matter altogether, but there do not seem to be any reports of any problems in the cinemas lately. Still...you are AYOR.