I actually think 13+ is too low for this film. Isn't it R in the US...?
The rating board is very permissive in our province (it's very rare that something is rated more than 16+). Often we see movie that are R rated in the rest of Canada getting a 13+ rating here (even some got a G one). Except for adult film (the kind I watch) almost nothing get the 18+ rating (one of the few I can think of is Wolf Creek and, to be honest, I don't have a clue why).
Personally I was more than happy to have a permissive rating board when I was a teenager...