I think we're missing the point of this whole thing.... we like to slap a label on the issue, called 'mature', when really it's about a few major things:
1. Is the person able to easily discern between fact and fiction? Real and fake?
2. Is the person able to understand that what goes on in the entertainment industry would not practically apply to real life?
These are the core concerns of a 'Mature' rating system. We don't care if the kids or adults are whiny douchebags or not... game behaviour is not the core of the argument. The system is designed to ensure that no one thinks that since they are able to kill someone and get away with in GTA, that they can do that in real life. They did this with the music industry and the film industry.
Bottom line, kids will get access to 'mature' content regardless of what checks we put in place. So what we should really be doing is making sure the message that entertainment does not equal reality across to these kids.
This all comes down to parenting. When I was 12, 13, 14... I knew and understood that what happened in the gaming world didn't apply to real life, same as in movies and rap videos

, but that didn't prevent me from acting like a douchebag when I failed or when someone cheated in a game I was playing. That type of maturity is immeasurable and only comes with experience.
So to define 'age' vs. 'maturity' is a moot point because what the system is there for is to protect the industry. So that irresponsible dad who leaves his gun cabinet unlocked can't blame GTA IV for his 10 year old shooting him in the leg.