While I agree with guidance in principle (if only because the sad truth is many supposed adults and parents are dolts) I do think game ratings are terribly awry currently.
Strangely, while films with dark themes and content seem to be rating lower and lower (12A Dark Knight, Terminator, etc) games seem to get rated very harshly (presumably a result of the whole video-games having a bad influence nonsense).
Most 16+ games I've seen in the UK are clearly terribly rated, and I have no problem letting my 12 year old and 9 year old play them. Probably the only games I restrict access to are any 18+ or other whose themes/content I don't believe they are mature enough to understand fully - and that's more about waiting until they can appreciate the title properly than worrying about some very unlikely negative effect.
Of course part of the challenge is children can vary wildly in their ability to handle certain content - ratings, etc. are an attempt to standardize something that somewhat defines such easy rules. Really a parent should understand their own child, and guide (and if necessary restrict access) their choices accordingly (as others have noted the rating would be better taken as a guide for content rather than a direct indication of suitable audience).
In the end though I think the ratings are almost redundant anyway in as much as very few people seem to take the least notice of them. The number of parents I know who have no idea what their kids are playing nor what rating the game even has (or why) is simply amazing.
Try to be reasonable... its easier than you think...
