I think it's 18 to enter, or at least that's what I've heard. Never really had any interest in going there.
You could always go to a hookah bar in Little Egypt (a neighborhood in Astoria, Queens). There are like 10 of them on one block (Steinway street and 28th avenue, or something like that). It's really a wonderful experience. Very relaxing, and very enjoyable. I've never taken the subway there before , but the V, G and R trains all go to Steinway and Broadway.
I'm not sure which part of the city you're staying in, but finding these trains (at least the V and R trains) are pretty easy.
The hookah bars are 18 and older though, but they hardly ever card, unless you look really young.
A couple of the workers at Egypt Cafe are also into metal, and they're in a folk band that plays at the club once in a while (not sure when their next show will be, but they had one a couple of weeks ago)
Don't rely on cabs, since it can be a pain in the ass to get one on the weekends, at least in the Murray Hill/Gramercy Park area.