you have all pretty much summed up my thoughts and approach to kids and gaming - my lad (14) has games older than him - no 18's though but they are assessed by me as to the content so language is fine (we have had boundarys / rules for that since he was 6) and if it will scare the bejeeezus out of him, I want him to enjoy the games not be terrified of them. Also it is strictly enforced that his younger sister is not in room when he plays them. He also likes a lot of metal music plus other stuff.
mind you the one game that did set him off into this alternate personality was Tekken 5- great shame as the reason he was playing it was because I thought it was a great game - we had it back off him it went in the cupboard and has not been out for about 5 years - he just kept getting that angry playing it we had to take ages calming him down - I also pointed out that was what he needed to tap into when sparring in karate and kickboxing... but he never does...go figure??