We went to the preview the other night. It is rated pg-13. It does have the start of a sexual encounter, they do get down to underwear, but it then fades away and leaves you with the impression of what was done. There is a lot of killing, vioilence, and language. It is pretty intense.
Personally, I probably would not take an 11 year old and a 13 year old would be marginal, depending on what you think he/she could handle. If he/she gets upset by violence and killing or scared easily, I would definitely not take him/her. Is there another movie they could go to while the rest of you see this one?