I am very flexible on ratings and whether I allow my 9 and 5 year old to view them or not.
My 5 year old has seen a couple of 12's, but many I have deemed inappropriate for him, same applies to my 9 year old.
The 12 ratings are, in my opinion more open to judgement than say a 15 or 18.
YANBU regarding CoD though, it is extreme violence in my opinion and as a game, you're not being 'spoon fed' you are actively involved.
Even though I'm flexible and don't always stick to the guidelines I was livid when my eldest son's friend's parents allowed my son to play Grand Theft Auto on a playdate... !
I had a friend who allowed their 8 year old to play World Of Warcraft, he begged me to allow his son into my guild, was never going to happen. His son ended up giving it up because 'people were mean to him' (naturally) and my friend blamed the game! Grrr.
When it comes on online activity I am very cautious, even some age appropriate games are off limits.