When I first saw this, I thought it was a horror movie (though I was 11 and somehow it didn't scare me one bit). Now I realize it's something called a...horror-comedy? Great movie, very scientifically factual, but I didn't find it scary OR funny any of the three times I saw it. (Ehm! Watch it...over and over.)
My rating: PG-13 for some brief creature violence, a scene of partial nudity, and some language.
I watched this when I was nine, and it didn't scare me one bit. But if you're scared of spiders, don't watch it. A scene of brief nudity (girl in shower), and some spider bites, but nothing worse.