Just because the censor stuck an 18 certificate on the box doesn't mean that children properly raised by parents with their critical facilities intact will get nightmares from the few red pixels on their screen.
What it does mean is that someone of a religious or 'moral' bent has complained loudly enough that a politician has passed a censorship law to appease teh said aomplainer, and that the censor has slapped that 18 certificate on the box in accordance with that law.
If you make your decisions based upon the bleatings of the likes of Mary Whitehouse then by all means don't buy the game for your kids. The rest of us may still be in possession of functional brains and be able to assess each item on its individual merits. We are much more likely to say to little Johnny, "no, you can't have that game beacuse it looks rubbish," rather than "you can't have it because you're not old enough".
Of course, what we really have to worry about is the day that some do-gooder decides to maken it an offence to buy an 18-rated game for a minor, in much the same way as buying alcohol for kiddies is an offence.