There's definitely an amount of controversy around the idea of children playing certain video games and rightly so. For a lot of people, it's a matter of parental control, of the adult researching their purchases instead of relying on the developers of an 18-rated game to make that game suitable for their young child. The same adult, however, says that it isn't so easy to exclude their child from something popular, especially if they frequently visit friend's houses. It's not just a case of shutting down access for your own child, but relying on other parents to do the same.
However you look at it, it doesn't say much for age ratings and a recent poll by Digital Spy suggests that as little as 29% of gamers even check the age on the front. This might be down to being over 18 and having no children/not planning to let them play (the situation that I'm sure many of us are in) or it might be down to not fully understanding exactly what that little number on the front means.
It all comes down to the same thing though: knowing what you're buying and knowing what's suitable. I say the same thing to people buying 50 Shades of Grey and complaining that it's a porn book: If you buy it just because it's popular, you don't get to complain about the content.
