You and Bubbakush, while I respect your opinions, are missing the point here. It is NOT the responsibility of the game maker to ensure games are age appropriate. That is what the ESRB or PEGI and various other organizations around the world are there for. They review the title and rate it appropriately. Same with movies, who may strive for a certain rating but don't have the final say. If an adult wants to take their child to a rated "R" film, there is nothing stopping them from doing so. They don't even have to go in with them, they can just buy them the tickets if they want to.
The game developers job is to MAKE THE GAME. Not to parent or raise our kids... or enforce restrictions on them. It isn't against any laws for them to play the game. It is the responsibility of us as ADULTS to act as such if a Child wanders into the lobby we are playing. Mute them or at the very least act like an Adult. Do I think kids SHOULD be playing? No, my kids wouldn't be allowed to. But THESE ARE NOT MY CHILDREN. As much as you may or may not like it.. you can't control how others raise their children.
Some of the Adults who play this game are far worse than the children. If you get into a lobby you don't like, or have someone you find offensive or just not desirable.. MUTE THEM. Move on with your life. When you have children you can set your own boundaries for their behavior. Online Interactions ARE NOT rated. Thus it involves PERSONAL RESPONSIBILITY. The Devs nor XBL or PSN or the Government are NOT in place to be Nannies.