I predict that this is all going to explode at some point soon with the Xbone / PS4 generation. We're reaching a point of photo-realistic animation in games, and that's a whole other ballgame to the cartoon violence of things like the first Mortal Kombat game.
Eg, I watched the trailer for the new Metal Gear outing, and whilst it looks fantastic it's very realistic and bloody brutal. There were scenes in the trailer that I felt a bit squeamish watching, and I've not been 11 for thirty years.
Nah. We have a proper, EU-wide, legally enforced (in this country at least) age system for buying games in the first place. Those in their 20s, 30s and beyond make up the vast majority of the video game market (by sales at least). The AAA games have budgets bigger than many movies. It's as laughable a notion that videogames are for kids as the one that bicycles are only for those who haven't got a driving licence yet.
Adults will continue to make games for other adults (although only about 4% of games actually get an 18 rating). Bad parents will continue to buy and supply unsuitable games for their kids, despite ratings and content info being clearer than ever, and every game system now having very good parental controls. Just like with DVDs, and videos before that. Good parents will inform themselves, set limits and make choices based on the maturity of their children.
fwiw, just as with film I think the implied violence and brutality can often be far more disturbing that that which is directly portayed. It's usually what you don't show that makes it worse. Good game designers, like good film directors, can make excellent use of that.