i put it like this, i wouldn't let a 9 year old watch 18 movie, so he can't play 18 rated games, such as GTA, COD, saints row and L4D. It works both ways, u wont watch a 9 year old watch an adult movie, so why let him do it if it's in a game? Of course, only u know how we acts with these things, but I'd be a bit ashamed if a 9 year old was walking round the playground swearing and talkin about his "hoes". Of course im only pointing out SR2 and GTA here, but the general theme applies throughout.
This is just me, and my opinion on the matter though. When I was 9 I wanted to play all the games like that anyway, so I can't talk much from the 9 year olds perspective, but I'm a little older, a little wiser by 13 years, and yes, more mature as it goes. Just take some control and say what needs to be said.
Some games are slightly more acceptable than others for a younger age. Mass Effect, being a Sci-Fi setting makes it easier to distinguish the difference in reality and fiction, though games set in "the real world" do come with conflicting values for the younger ages.
Just think about them games and what they say. I realise I'm getting a bit "Guns don't kill people, Video games do" on this subject, and consequentially ended up with quite the post, however, just take the issue to your parents, show them the game and put the responsibility in the hands of the parental units, since it is them that supply the games. Just give them an idea of what they are letting the 9 year old in question play.
Hope I managed to shine some light on this =] And sorry for the mammoth postage there guys lol