Here's another good one: "Are there any health studies about what exposure to violence or horror does to childhood development?"
There is a reason that rating systems exist and that we shelter children from these things.
The pre-rebuttal that you posted "this was common in my childhood" is no indicator that this was a healthy behavior for you or the masses.
Ratings are very criticized by artists, eg as being fueled by conservative moms. For example, in the USA, movies with guns and explosion can be shown to younger audiences than nudity - seems very illogical.
Also, some anecdotes: lots of my friends were into GTA as kids, ie early teens, and turned out fine. Comparing to kids who didn't do so well, I consider the most important factors to br family, education, and finances, not violent multimedia.
With that being said, I'm sympathetic to limiting internet access due to communication with strangers, and extreme content (eg violent rethorics that appeal to action, not fantasy violence).