Every person is different but if a parent lets a young child play an M rated game that includes intense violence in some cases it can lead to behavioral problems and make them aggressive. The reason we have a rating system is to let parents know what content is in video games and what age a child should be generally before being exposed to it, I mean parents obviously have free choice to introduce their children to whatever they wish, but as a child's brain develops one of the things it creates over the first 16-20 years of life is what is appropriate social conduct and how they should act around others, and exposing kids to cursing, intense violence, gore, ***. It's not good for a child socially... I mean some kids can take it but that means their parents teach them good social conduct and to define the difference between games and reality which is where kids get confused they think hitting people and cussing and acting sexual is OK because they see T and M rated characters do it and think that its OK because they see these characters react to one another.
Kids need to develop 2 things before playing M games; decent social conduct and a concept of reality and fantasy, which takes years, you might mature faster than other people and your parents may know this and let you play them at a younger than suggested age but that's their choice. The responsibility is fully on the parents.
We have a rating system so parents can decide not so game makers can walk door to door and instruct parents on what's right to play at what age, parents know their kids and are able to make the decisions themselves and just because a game has mature content in it doesn't mean you get rid of the game it means you don't show it to kids, just like adult TV shows, or animes, or books.
My favorite quote for this argument and other censorship arguments is by Mark Twain... Censorship is not letting a man eat a steak because a baby can't chew it.