Age restrictions are a guideline, and up to the parents' judgement. I think most kids recognize that what goes on in a game like GTA is not acceptable behavior in the real world, and few would want to replicate it. And the ones that would, in a lot of cases, their parents have already failed.
All the same, I don't think it's an ideal game for kids, and agree with the age restriction. Not because they shouldn't play whatsoever, but that it shouldn't be easily accessible. In the same way, kids can watch R rated movies on TV, but usually not until after 9 or so (I haven't lived in Canada for 10 years, but last I was there, even on cable, they didn't show 'adult' movies in daylight hours). If a 12 year-old made COD and GTA etc the majority of their entertainment, I think they would suffer negative development: a lack of empathy, a belief that killing is cool, stunted emotional development... But it would depend how they interact with others, what else goes on in their lives. I know how kids are. They find what they like, and can totally wallow in that sort of thing. It's much better to make this sort of thing something they have to work to get.
<in my case, because of the era, it was pornography that was hard to get. Games were pretty tame/abstract. I acquired a few mags here and there, but if porn was readily available, I would have never left my room. I don't know how the 13-year olds today handle it. But that's another topic.>
