Niceguy2 makes a good point. What are your initial reasons for saying no?
I personally think it depends in the child.
It never ceases to amaze me that if you are talking about child development generally, people always say children develop at their own pace and you as a parent know them best.
As soon as some authority or another puts some kind of arbitrary age restriction on something, some people see it as a line that must not be crossed, whether that is game / film certificates or weaning age.
If you are saying no because you think your son is too young / immature to handle the violence / swearing etc and will copy the swearing and get nightmares then don't let him have it.
If you think it is because he will copy the violence, then you are being a bit precious.
If you think it is because he will spend too long playing it then you shouldn't let him have a games console at all because the same could be said for any game.
I missed the gaming generation by a few years, but my parents (and my friends parents) were quite happy to let us watch horror films from the age of 12/13 - we spent many a sleepover watching the Nightmare on Elm Street Films.
My brother used to watch Aliens and Terminator from quite a young age and also used to play Halo when he was 14 (although that was only rated 16).
This was allowed because our parents knew we could handle it.