Well I'd say yes and no. It all depends on the content of the game. But all in all I think the ratings are on those games for a reason and parents should be cautious. It's the same as movie ratings the movies are rated for a reason, I took a friend who was 18 to see the movie Saw last year, (it was rated R) it's safe to say the movie scarred her for life. She ended up having a major anxiety induced freak attack after the movie, and I can tell you this it was not in character of her to act like that at all. Me being totally desensitized found the movie to be very funny. My friend on the other hand, who scares easily, wasn’t used to that kind of gore or content and it affected her. She actually ended up at the hospital in the morning with a severe tension headache which she had never gotten before. I’m not saying most people would react to things like that, but you’re kidding yourself if you think certain things in video games don’t have psychological effects on some kids/teens. Where I live they have actually changed the ratings on movies and now any movie that is rated R nobody under 18 even with a parent or guardian is allowed in the movie period. And they are starting to crack down in video rental stores or retail stores and charge places who rent or sell to the underage. And trust me they used to sell them to anyone, I rented and bought rated R movies when I was 12 with out being asked for ID and I looked about 7 years old then.
And I know this is different, but just consider this, how many parents who actually care about their kids well being go out and buy their underage kids alcohol and cigarettes? One might rot your body, but the other can rot your mind, and that’s almost just as bad.
With that said, I’m not qualified to answer what affects it would have on someone long term. I myself started watching horror movies, violent movies and playing violent video games when I was like 6 and 7, and nothing i see on tv, in movies or in video games fazes me. I don’t jump at scary parts in movies, I usually end up laughing at gory parts where body parts are flying around. Same with the video games. But I still react differently to the real world, seeing gore and death in real life and up close still affects me. Maybe I can just tell the difference between what is real and what is fake rationally.