I reckon that what kids see and hear can affect them. Every child is different though and the impact is lesser or greater depending upon their disposition and the viewing material in question.
In our house we have kept it simple for the 12yr old lad. He's a bit prone to being influenced by certain things that he watches and plays.
Film/game classifications and ratings are there for a reason. Stick to them.
If Mum or I say 'No' or 'Yes' then that's it, as some discretion is needed as the above can be pretty illogical.
Internet is accessible but through a filter (can't recall its name atm) that is so stringent it's ridiculous. We'd rather that than the other end of the scale.
All passwords/pin numbers to be known by Mum or me for any site/phone and we have access at any time without warning.
and...
NO you can't have a Facebook acc until you are 13!!
As parents, We have the final say.
This ties in with the fact that we are open and welcome any questions or discussions about anything.
It's very hard in this day and age to control what goes into your child. IMO it's important. Everything is so accessible and this is a good thing and a bad thing.
Since he's started secondary school we have noticed this become an even greater challenge with the advent of increased freedom of opportunity and knowledge shared with friends at school.