I think it's okay middle school and up, but other than that, just keep them from E-T rated stuff. It also depends on the kid. I'm 13 and really mature for my age but I can't play any M rated games, so I'm kinda stuck. Don't do that to your kid, EVER.
I think it depends on the age and maturity. If I were a parent I wouldn't want my 12 year old playing an M game, but at 16 or 17 (depending on maturity) I think it would be alright. I mean sex ed is taught before that age, you see murder and crime on the news everyday, and bad language.. well that's all over the place.
In opinion, it is not advisable for them to be playing such games because they will have corrupted minds. Depending on the kids' age, you can get them non-violent games like basketball, car racing or any game that they might be interested in.
Absolutely not! Would you take your child to a part of town that is full of gang activity and prostitution, let him out and say; "Have fun"???? Our children are growing up way to fast. They need to be kids. Draw the line as a parent and don't give in to "their" peer pressure.