I Am Legend, But Am I Kid-Friendly?

Lots of discussions with various parents over holiday vacation about who might want to see "I Am Legend." I enjoyed it thoroughly -- I'm a sucker for dystopian, apocalyptic science fiction, and Will Smith carries the loneliness of the movie with his usual hard work and charm. But I have to admit I was surprised, after leaving the movie, to look up the poster and realize it was rated PG-13 instead of R. To me, the bloodletting, the shock value and the general atmosphere all screamed R, not a lighter PG-13. I had considered taking my 15 year old daughter, who has seen many R movies for a film class in high school, but I was glad she couldn't make it that night -- I think the violence and nightmarish qualities would have disturbed her. Then again, my 15-year-old nephew loved it and has seen it three times. Do boys take all these things less seriously? Have videogames enured them to some of the worst spots?