Gribble: why do you think age-ratings are "random". Do you think they pick the titles out of a hat? I'm confused by your point.
I don't think anyone has claimed that they truly believe being 18, as opposed to nearly 18 makes a vital difference either.
But as for universally trusting Parental Judgment. Pah. Don't make me laugh. I play computer games. Lots of them pre-children. Somewhat less now, due to time constraints. (Same goes for books, too, incidentally.)
The majority of people purchasing games for child, in my experience, just say "oh it's a game". Parents who would never let their children watch a violent film, will buy an 18-rated game, without concern. Frankly, games are interactive mediums, and I would think that directing violence to occur on a screen is less significant than watching someone else do it on a screen.