This is an interesting issue for me, because while I was not allowed to play M games until I was 16 or so (starting at around 14 my parents would take games by a case by case basis) my brother, who is 4 years younger than me, essentially got a free pass starting when he was 14. It bothered me a bit, but I think I understand why now.
First off, anyone that lets their 12 year old play GTA 4, under any circumstances, seems to either be ignorant of the facts of the game, some other external factor, or just unaware. Either way, it shouldn't be happening. Now, a 12 year old playing Splinter Cell: Chaos Theory? Less of a problem for me.
Also keep in mind that these are all guidelines, and while we may disagree with their decisions, these are not breaking any laws or really violating anything other than good judgement. In fact, us gamers have fought tooth and nail against people like Jack Thompson, who try to place purchase restrictions on M and T rated games. We just don't like anyone messin' with our freedoms. And it is the parents "freedom" to let their child play M games.
Let me get back to my main point. I was mad when my parents gave my brother the free pass. Why did he get it 2 years before me? I wanted to play Oblivion or Morrowind when I was his age but they wouldnt let me, but now he is playing Skyrim!
The main concern surrounding children playing these games is what they take away from them: what is acceptable behavior (running over hookers and taking their money an obvious no-no), desensitization to violence, and general attitude. What I discovered over the past year or so is that my parents were not waiting for some arbitrary date - they were waiting for me to, as cheesy as it sounds, be mature enough to be able to separate reality and virtual reality. When I was at that point, while they might not like the games I play (my mother especially hated Fallout 3, for example, and even though it is tame compared to some games, cannibalism freaks the hell out of her and she saw it was an option when I was choosing a perk) they understand that I can keep it where it belongs - in the game. So even though my brother was younger, he saw me being able to maintain this separation - and he undoubtedly has as well (although there was one rocky period.)
I think my real point is that the ESRB are "guidelines," not laws, and should be treated as such. While they are a good way to take a first glance at something, each game should be evaluated individually, just like a movie. I am just astounded by the number of "every parent who lets their kids play M games before they are 17 are stupid" posts in the thread. This mentality is damaging on a personal level as well as a societal one.