Ratings on video games are very political.
Because games are traditionally the domain of children, a lot of negative publicity/hysteria has linked video games to violence in children, there has been a lot of pressure to censor and strictly rate them.
Video games join the long line of punk rock, video nasties, dungeons and dragons and other pastimes that have been maligned by adults that have no idea what they're about.
That said, some games do have violent themes and I wouldn't particularly want small children playing them. But I've played most of the controversial ones and really haven't found them too disturbing. Most of them sound bad but in reality are pretty cartoonish. I wouldn't have any qualms about an intelligent, well-adjusted 13 year old gaming.
And I watched one clip from Saw once and was really upset and freaked out for weeks afterwards so I'm a delicate soul sometimes!
Disclaimer: I have not yet played the new COD. Potentially, a video game can be disturbing and terrifying. But in my experience, they're usually not, no matter how they're touted. Mostly, you're too caught up in the skill to be worrying too much about the 'violence'.