I'm not sure about others, but playing GTA 1 at 10 years old didn't make me a serial murderer as far as I know, nor did it offend me to shoot people's heads off on the PS2 with GTA III later. My parents did actually return this game though, they thought it went a bit far. Or my father did, my mother never got to see it, probably for the better.
Earlier, I think at about 7-8 years old, I played Command and Conquer 2: Tiberian Sun. I think it was 16+, might have been 18+. Yeah you really could see a few pixels blood for like two fifth of a second. Now that made me a real dictator, and afterwards I was screaming under the table. Sure...
Then the last game I really played a lot is OpenArena, mostly a Quake 3 copy. So with real pixels resembling some 90's version of blood and gore. Must say I never noticed it until I started playing a bit more pro, and I noticed the gore was actually in my field of sight. Then I set com_blood 0 and my visibility was improved... I was about 15 when I started playing. The game is 17+ but I only noticed that when I was 17 (I'm 18 now). That, too, didn't quite offend me.
So WHY do I ask? Why are we even bothering to print these things on the covers anymore? It is a very slight indication of what age category the game is for, but much more useful would it be to put on the covers something like "Blatant sex" or "Dull game good for children under 10". Now that won't happen, but the equivalents wouldn't be such a bad idea. Then they could stop making judgments of which size fits all (since you can hardly put up a personality test in the shop before getting an age recommendation).
Also something people apparently fail to notice how it's called an age recommendation. Here at least. Still though, as 15-year-old you won't be able to buy a 16+ game. Like the expansion pack for Command and Conquer 3. It doesn't even contain blood or sex or anything! Maybe in the cut-scenes of the campaign, but I never even played that (skirmish and online ftw). How could they know? They can't! So why make anymore judgment than a recommendation?
Moral of this rant: Parents: Don't you dare looking at the age rating! If it says what's in the game, like drugs or violence, that might be of some use. Best would be to just watch a gameplay trailer someone put on Youtube, then make the best decision for your child. Can you really trust other's to make the best decision?