I believe the first Mature rated video game I played was at age 8 when I gave "Resident Evil 2" a chance. It was a harmless Mature rated video game since you are only killing zombies and other infected. My parents weren't strict and they let me play "Grand Theft Auto III" when I turned 11.
I think I would let my kid play a Mature rated game as long as I talked to him about that he would never do the stuff in video games in real life (which I already know they wouldn't) because I knew lots of kids back in the day who played Mature rated games and grew up to be responsible adults with no criminal record. I don't think I would let him play Grand Theft Auto V because it has gotten much more offensive than GTA III did. We see sexual intercourse, torture and naked strippers. Whereas GTA III wasn't as violent, had no f-bombs, and no graphic sex or nudity of any kind. I definitely would not let my kid play this game until they were at least in high school.
The way i see it, if gun nuts down South will let kids hold dangerous guns, then I think letting them play violent games is infinitely safer. Parents are f***ing irresponsible for owning a gun with a child in the house, much less let them hold it. The only way a video game would be dangerous is if you break the disc and use the broken pieces as a sharp object.
The first M rated game that I can remember playing was probably Mortal Kombat for the SNES. My brother was home from collage for the summer and wanted to play Street Fighter with me, but all our local Video store (god i miss those) had was Mortal Kombat.
I don't think my parents even knew that video games had ratings and I'm sure they wouldn't have cared anyway. I was always allowed to watch pretty much whatever I wanted short of full on porn. M rated games back in the 90's were nowhere near as violent as modern day games anyway. Yeah there were the fatalities in MK and plenty of gore in games like Doom and Duke Nukem, but they were little more then pixilated blobs of red not the High Def murderfests kids are exposed to these days.
EDIT: Forgot to include my age. I was 7 when I first played Mortal Kombat
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I believe it was the original Mortal Kombat on the SNES. I remember my cousin thought he was real cool whooping on me since he had already had tons of practice, until I learned how to play Raiden....Then it wasn't so fun for him anymore, lol.