My parents let me play M rated games when i was younger because they knew i wasnt stupid and wouldnt take them seriously. (I mean when one game i buy is GTA and the other is spyro the dragon you cant really think im taking the games too seriously right?)
Obviously though, all children should be monitored on what games they play and how seriously they take them.
Many parents are careless and don't mind about the health of their children. Or, they just spoil them too much. Basically when an adult allows their young children to play a rated M game, it's like they're begging to brag about how cool they are for letting their kids play whatever they want. =_=