Games in particular would be easy for younger kids to get if they knew an older kid who could buy them. When I was 18 I would have bought anyone a game because I like games and don't seem them as harmful. Maybe that isn't the best thing to do (I wouldn't do it now probably) but I definitely would have done it at that age.
At what point do we decide that someone is 'mature' enough to play a violent videogame? Is it a different age than when we think they could use drugs 'responsibly' (in a world where they were legal)? A more obvious example would be alcohol. I would let my children (n.b. I do not currently have children) play a violent videogame, but I would not let them drink a six pack. I see the drugs/alcohol as putting them more in direct physical harm (most likely where they would hurt themselves, or overdose, etc.) than violent videogames. People thankfully can't overdose on violent videogames (except the people who died in Korean internet cafes playing WoW, but I think that's a different topic altogether).
Given that someone can't physically hurt themselves with a violent videogame, I would allow a much younger child access to them than I would drugs or alcohol. We could argue whether or not there is any mental/emotional damage, but I think we can agree that whatever mental/emotional damage there could be from playing violent videogames at the age of 12 as much as one wants, it would be much less than the mental/emotional damage of letting a 12 year old have unlimited access to drugs.