No, your kid is probably not going to be "irreparably mentally harmed" from being exposed to violent video games too early. But there is such a thing as exposing a kid to shit that they're too young to handle (or understand). And that's not a good thing.
A kid's innocence is a precious thing, and it's a shame to squander it too early. That doesn't mean I want to have my 12-year-old still believing in Santa Claus, but I also don't want him introduced to the ugly world of violent crime, drugs, and prostitution while he's still in kindergarten either.
You probably have the best comment here so far.
It all depends how you support your kids. If you don't have time to set the moral standards for your kid (because you're a single parent and gone all day - keep this in mind when watching Dr.Phil in the distant future) and buy GTA5 to basically babysit your teenager - you're in trouble.