That's what the age rating is for, the game would simply not be meant for them. Does not take any research, just requires a parent to look at the little box in the corner of the game cover.
No, that number is according to who?
After all, Road House has a +18 sticker. AEROSMITH's dvd has a +18 sticker. But if it's a "religious" movie, even if it's gory, that's fine, please do take the kids along... Oh well.
At least here, I'm pretty convinced that ratings are thown in the air and fall wherever. It's the only explanation (the Road House and Aero examples are UK editions, I think). Probably the same happens with the games..
No, parents should actually get involved, not trust others to do it for them (based on what?).. Besides, like grownups, kids are different from each other, and what's ok for one isn't for another... me thinks.
(Though I have NO IDEA how to decide that.. but hey, I don't have kids!)