I don't think that the ratings system should be based as something that a robot could do. The content of a game should be more about the general atmospehre, and more on rule of thumb than an exact thing.
I'm thinking of a Eight step system
Infant+
Child+
Child-Tween+
Teen+ (Halo would be here or)
Teen-Mature+ (here)
Mature+
Mature-Adult+
Adult Only
I'm thinking of a Eight step system
Infant+
Child+
Child-Tween+
Teen+ (Halo would be here or)
Teen-Mature+ (here)
Mature+
Mature-Adult+
Adult Only