Expanding on your question Dadwashere, if you're talking about specific conversations with them, I've had a few with my DS, who is a teenager. (And sometimes engages and sometimes cringes and says "Shut up Mum, I know all this! 
- depends what mood he's in.)
The tack I took/ take is that moving images are powerful, which is why there's guidelines for what films you're allowed to see at what ages - because seeing something moving on screen, is different from reading or a still image and may have a disturbing impact and that's why they restrict children's access to certain films.
Also that his sexuality is his business and he has ownership of it and because he is very young, it is not yet formed and it's important that no-one else comes along and forms it for him - not an abusive adult and not a porn merchant. He will be subject to all sorts of messages about his sexuality (indeed, inevitably has been, we all are from the moment we're born) and those messages will inevitably influence it, but it's a good idea to have a conscious awareness of all those external messages, many of which will be from people who have a very limited view of sexuality and a vested interest in ensuring he sees and accepts their view.
If he accesses porn on any significant basis when he is very young, he is in danger of having his sexuality owned by people who don't have his interests at heart, who have a deeply misogynist attitude to women and who are pretty all-round shit bags. Does he want them to control his sexuality, or does he want control of it?
In the same way I've spoken to him about smoking - do you want tobacco merchants to control your health or do you want some semblance of control yourself?
Obviously I've done it in age appropriate language and acknowledged that no-one has 100% control of these things - unpicking how much of our sexuality is innate and how much is learned, how much of our health is in our own hands and how much not - no-one can unravel that. But that's my starting point for stuff like this, I don't know if that's the best way to approach it but can't think how else to do it.