I held out for a long time on even having XBOX in my house, because I felt better about games offered for kids with the Wii. Simply put though, your kid will be exposed to WAY more than you even want to know about on the school bus, in class, in gym, in the mall, at movies theaters...my boys wanted HALO, and it's only on XBOX, so I finally caved and bought it...they now own nearly every first-person shooter game you can name, and I kept telling myself initially "they're killing zombies, it's OK", or "it's just shooting aliens" so I'm good. I found I was trying to justify to myself the reasons I was allowing them to play these games to feel better, and my ex doesn't approve to this day. However, I'd much rather know what they're playing and have the opportunity to see it for myself over just wondering, and trust me, if they aren't playing it in your house, they WILL be playing it at a friend's house, and you can't control that. You may not ever even know he's playing it elsewhere...
So, I've had talks with both of my boys, now 11 and 13, about violence they see in video games, and how some kids don't understand the difference. I have watched them take to guns like a duck to water, and OMG, they are great shots right out of the gate, first time, ever, hit-every-target and even impressed the volunteers at the local gun range saying "Ummm, I think these two pass!" So yeah, some may find it irresponsible to buy such games, but I find it even more irresponsible for any parent to ever think "Not my kid" because your kid will do what others are doing, they will cave to pressures, they are exposed to everything "cool and hip and new and exciting" daily at school, and there's nothing you can do to stop that exposure short of home-schooling and duct-tape. I'd rather be proactive and completely involved in what they're playing over pretending it doesn't exist. I'd rather have the open and honest discussions about what they're doing and seeing over just ignoring it and hoping it never comes up. I truly believe the kids that are denied things like this, or the ones whose parents don't take an active role in what they're playing, are the exact kids that do develop problems, that blur the lines of what's real vs. what's not.
I do find it curious, though, that most of society, myself included, has decided violence is better than sexual content...I am finding that even I stop at "contains nudity or sexually explicit content" in rated M games, and won't buy 'em for my boys, but shooter games and blood and violence is OK? I struggle with it, but yeah, that's kind of how society is. So, as for GTA5, nope, I won't buy it, but I have purchased rated M games over and over for my two for 2 years now.
I recall being just like you when my boys were young, wanting to wait for just the right age to do more age appropriate things, but the simple fact is 1) I look at things I was doing and what I knew by 11, and my jaw drops and I wonder if my boys know the things I did...and 2) you cannot control what your kids are exposed to once they hit school, no matter how hard you try...you'll see as your son gets older. I trust you'll also see the benefit of being more involved in what he sees over pretending it's not there at all. Trust me, it is, and trust me, he'll be surrounded by it even when you've convinced yourself he doesn't have a clue...