I was probably a little young, but I was a pretty mature kid, so I don't think it mattered much. I was around 12 playing GoldenEye 007. The graphics aren't that detailed, so the violence isn't really that bad. Before that I was playing Command & Conquer: Red Alert. That is an RTS, so the violence is a bit removed. The earliest violent game I played was likely Doom II. I was probably playing that around 10 or so. I remember my Dad buying that for me in a Best Buy. My parents figured I could handle it.

I was playing Mortal Kombat when I was 13, and I played Leisure Suit Larry when I was 16.

At no time in my life did I enjoy blood in games more than when I was a kid. In anything really. Games with excessive amounts of blood were automatically awesome.

Pornographic was great too! But not pornographic with blood and/or gore (I was a crazy kid but not that kind of crazy).

I can vouch for this. A lot of the games I enjoy are T or even E games, though I still like some M-rated games, including Mortal Kombat. My enjoyment of them doesn't have anything to do with blood or gore, though Bayonetta wouldn't have quite the same punch if it didn't have the blood and just short of complete nudity.

All the time.

GTA III, Mortal Kombat 2, GoW2, Syphon Filter, Twisted Metal 2, etc.

I played Just Cause 2 for the first time when I was 15. Not that much younger than the recommended age, but still. I screwed around for a few minutes in the PS2 GTA games when I was about 12.
I played plenty of T-rated games before I was 13. Smash Melee and Brawl, the first two TMNT games on GameCube, and some others.
I do think there is a problem with kids under 10 playing M-rated games (Call of Duty, GTA). I wouldn't want it to happen if I kids, but it's up to each kid's parents. Most kids around 13 can probably handle M-rated games, and almost all probably can around 15-16 years old.

Plenty of times

Yes, I was playing M rated titles as a kid like GTA Vice City, Halo, Jade Empire, God of War etc…

Wolfenstein3D, DOOM, Duke Nukem, Carmageddon, Leisure Suit Larry 3, 5 (there is no 4) and 6, WET: the sexy empire, Biing!, Geisha, Spellcasting 101 &102...

16 bit games and even most 32biters were pretty tame bc of limited graphics anyway. I remember watching the Alien movie as a kid and that was intense.