GTA IV is NOT age appropriate. I'm 32 and the amount of violence and sexual content is inappropriate for me, but still love it. MGS 4 is a shooting game, but if you liked Assassin's Creed, then you will like MGS 4, as they are very similar: not fast paced, stealthy, take your time and find stuff. Also, the blood can be turned off on MGS 4. While it has a great story, the violence isn't nearly as bad as GTA IV.
I too, do NOT like shoot 'em up games (or FPS, First Person Shooters), but loved Assassin's Creed and Bully and have played every Metal Gear since day one. Assassin's Creed, Bully, and MGS 4 are all very similar games play wise, the only difference being MGS 4 online is a straight shoot 'em up style, just like any other FPS game out there. I've played all of 10 minutes online and am fine with not going back.
As far as other games like GTA (sandbox style), there really aren't any out yet for the PS3. Oblivion is that style, but is an RPG (Role-Playing Game), however, plays more like Bully and GTA do with the fight as you go and do what missions you want to when you want to, or just roam around. Also, unlike regular RPG's where you are thrust into random battles while walking around, you aren't thrust into random battles while walking around in Oblivion, and can see all the enemies, just like GTA. I would recommend Oblivion as it is a pretty long game and fits the style you are looking for.