Originally posted by Angelus the Vampire:
Can`t really help you there.
As my kids used to accompany me to sci-fi conventions when they were young, they were able to tell the difference between fact and fiction from a very young age. Meeting, in person, an actor who played a mean baddie on TV, but was a great laugh "in the flesh" tended to make them realise that what was on the box wasn`t real.

However, while I would occasionally allow my kids to watch programmes aimed at an older age group than they were, I wouldn`t let their friends watch those programmes at my house. And I did create holy hell when my son`s friend`s mother allowed him to watch a 18 cert horror film when he was 13.