you DO have to be 18 to legally enter the shop, but most shops won't id you at the door or anything, as long as you look like you're of age. of course, if you tried to buy anything they'd card you.

also, be careful what you say inside a headshop, the ones i've been to have signs saying that they will ask anyone to leave if they talk about anything other than tobacco, for obvious reasons.

and one last tip, remember, its a water pipe, not a bong