YANBU, this is something that really pisses me off.
It's not even about the games and the age ratings as much as it is about the fact that I don't want to send the message to my children that it's ok for them to just ignore rules that have been set by people that are supposed to know what they're talking about.
If they are led to believe that it's ok for them to play 18 games at 13 years old, then it wouldn't be a huge leap for them to make to assume that the age for smoking is set too low, the age for buying alcohol is set too low, there is no need for certain drugs to be illegal.
I appreciate I will sound like I'm over reacting, but I do think that that DVDs and computer games are the thin end of the wedge.
I have allowed my dc to play one game that they are underage for (can't even remember what it's called now!) but they did the whole 'it's not fair - everyone else is allowed to play' thing. And it turned out they were right, quite a few of their friends were allowed to play. I made them explain to me why they thought this particular game was rated too high, and they were able to find lots of opinions on the Internet from sensible people who also thought the game was rated too high. They had to explain why it was given the rating it was, because I wanted them to understand that rues are there for a reason and they have to be respected, but also that if they have good reasons for challenging those rules then that's ok too.
But I resent that I am the one that is made to look unreasonable in the eyes of my children because of other parents making choices that go against well thought out guidelines.