I'm feeling around in the dark a bit here... but what worries me about kids playing GTA online is the people they come into contact with.
The content is M for mature, sure, but kids talk about this at school and bring up anything they want to on their mobile phones anyway, so it's not that, but what they don't do is swagger around with mature adults who have a real life experience that is completely different to their own.
It worries me that one of my boys could be gang-bullied online, subject to abuse, taunting, that sort of thing... and that's why these kids scream online when they get dominated. I talk to my wife about it and we don't want our kids being subject to this sort of thing... not just yet. Nor do I want to see any other kid being subject to bullying by older boys/girls or adults for that matter.