I'd agree with your sons, stressedHEmum, the ratings systems for games is very poor and flawed.
DaddyJ, you're right, it's not possible to pre-watch/read everything. I tend to skim what my children are reading and keep a vague eye on what they're watching/playing but I feel that what is more important is to keep an open line of communication and for them to have an awareness of what they are comfortable with. If they see something that makes them unhappy, we can talk about it and take steps so that they don't have to experience anything similar again until they feel able to cope with it.
Of course, it's not a perfect solution - once they've seen a frothing flesh-devouring zombie, it can't be unseen, but that's why it's good for them to be aware of their own comfort zone.
This really is far more important to me than saying point blank, "No, you cannot watch/read/play that" which just shuts down communication imo.
Also, I agree with previous posters about what is upsetting being so individual. I remember as a child, my stepfather watched violent Schwarzenegger movies all the time which didn't bother me one iota. Meanwhile, I was most upset and traumatised by an episode of CHIPS with a sabotaged car brake! (And don't even get me started on Casualty...)