I often play MW2 with my best mate and his 11year old cousin. All 3 of us are pretty decent players (the cousin being the best, K/D 2.1) admittedly me and my mate have a moan with the odd "bloody hell" thrown in but the 11year old swears alot and gets stroppy everytime he dies even when everyone tells him to keep a lid on it.
I would say the problem is letting young kids play games which were made for people older than them, simple as.
I would say the problem is letting young kids play games which were made for people older than them, simple as.