Your mother probably doesn't think you're mature enough for games like that, although, I could be wrong and she could just be trying to feel good that she's denying you them because of statistics and numbers.
If you think you should have them, why don't you buy them yourself? Think about that, it's because you're not old enough, if you want the games, you have to be cunning and make sure that your mother thinks you're mega mature for a while.