Sadly enough parents aren't educated on the subject. I have a good example on this.
I was out to pick my boss up, and with me I had his son. Anyway, since we had a little spare time before picking him up we went into a game store and had a look around. He focused his eyes on a game we all know - GTA

Anyway, he said it was ok for him to buy this game and that his parents had given him the OK to buy it. I suspected something was very wrong seeing how any parent would allow their 13 year old son to buy a game rated 18+. And my suspicion turned true, once I told him the game was rated 18+ and contains violence he was denied the purchase the game by his father.
The one thing I really want to bring up is that he has the previous GTA game home on his shelf and that one too has an 18+ stamp on it. The difference, they didn't (or still don't) know the characteristics of the game and went with their son told them, i.e. it's not really that violent and it's suitable for my age.
So I call it lack of education from the parent that is the cause for many people getting their hands on games there are not meant for their age. They simply don't know better than to go with what their kids tell them. And in this case he assured there wasn't that much violence.
He got a little pissed at me for calling his father up. But I don't care because I was doing the right thing, and that is IMO not giving him that kind of game.
It all boils down to the same thing, parents that aren't educated of what games are, and parents that don't do anything about it. Should I have a son or daughter myself I would never supply them with the game. Should they acquire it without my knowledge (which is very likely) I would as soon as I saw it most likely remove it and tell why I think what they are doing is wrong. Most of the time there is a good reason why a game is rated 18+, or 15+ or whatever.
So these days my boss asks me questions all the time about games since I am somewhat "into" the thing and I give him the background of the games such as game play style and all that. My boss is very strict with what he exposes his children to, and generally shooters end up on the block list by himself, after I give him the background of the game.
So a good advice for parents would be to make use of gamers of higher age to get a general idea of what a game is about and if it's violent and such things. Ask them what they think of the game and if they would recommend it to a child of a specific age. Or better yet start playing the games yourself for the best possible way to tell if your children are up to playing the game. After all, you should know your children best. If you don’t know where to get the games you can always start subscribing to a game magazine and get demos on CD/DVD, and best of all; reviews of games that should give and extensive insight of the game.
Now, don't mix what I have said above with anything like I blame the games for turning the regular choirboy into a bloody killer, because I do not think computer games are more responsible than the general movie with violence in it. The violence is something that’s been there all along, it has just changed slightly. In the past we used fists to knock our opponent out, today we use weapons instead. Such as sniper rifles, automatic rifles and similar types.
So society always changes, we always evolve to use technology available.
The games may have something to do about people turning out the way they do, I won’t deny that. But it’s definitely not the game developer’s fault their product is used in the wrong way. And they should never ever be liable for any damage caused, nor be open for people to sue them due to this. This would be comparable to people suing weapons manufacturers as soon as someone is killed with their weapon. And as far as I know, that’s not occurring very often.
So once again, it’s all about education. Educate your children, why some games are bad for them to play, and that they are not real at all. In real life bullets kill, in a virtual world they do not.
Alright … enough rambling from me for this round, I provided some mind candy for you to reflect upon and got my points of the matter out to the public.