I played more violent things than DMC when I was 12, and I didn't become a freak. :P I don't think my parents were completely ok that I played games like MK, but they knew that I was mature enough to know that it wasn't real. To me, that's the perfect model, although it does require some parenting, which is too much work for some.
To me the ratings are definitely a guide, however, for those that aren't well versed in the world of games (which I'm assuming I'll grow into as years go by).
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