I played all sorts of inappropriate stuff and watched things I wasn't supposed to watch. I understand your concern but frankly, it's none of your business and I know I would haven't been grateful if I hadn't been able to do some of the things I did as a child that would be deemed inappropriate.
I cannot say for sure how an early exposure to guns, war and violence in videos shapes a human being but I honestly don't believe in a meaningful impact. It had none of me or anyone I know of, for what it's worth. Although I understand you need a bigger sample size to draw conclusions.