There's nothing wrong with children playing games at an early age. Schools use educational games to engage kids and have them learn interactively. But the content is key.
Children until a certain age are sponging off the people around them for behaviour models. If most of the behaviour they observe is in video games, then yes there is an element of social withdrawal, but I think the result would be a different sort of social withdrawal if they were spending 6 hours a night locked in their room playing Dora the Explorer Read-Along Adventure as opposed to Manhunt.
This can't be used as an argument against R18+ gaming classifications. Well it can, but it's red herring used to get people to argue about whether violent games make kids violent (not yet proven conclusively), distracting them from the fact that R18+ games aren't meant to be played by children.
So yeah, I think children shouldn't play violent games or any game that models undesirable behaviour. Adults, however, are responsible for their own choices and should already know what's okay in real life and what's just to be left to the realm of fantasy.