Compared to some of the things that are out nowadays, 15 seems fine. Evil Dead II doesn't have a woman being raped by a tree, either, whereas the first one does.
Saying that someone can only watch this movie when they're the same age as someone who can watch a woman swallow seventeen loaRAB is a bit much. It's gory, but it's more cheap, nasty and funny than it is sick and disgusting.