I was recently interviewed by the lovely Natalie and Jacqui from "If Only They'd Told Me" about the work we are doing. One of the questions I was asked was around what age we should begin conversations with our children. After all of the research and talking to the experts and communities, I really do believe that we can start the process of keeping our children safe and being alerted to our children being exposed to online pornography, as young as three. If they have access to a smartphone, iPad or tablet, have siblings with access or friends with access then we need to say something.
And don't worry you can still keep it age appropriate. Simply explaining that they should come and tell you if they see any pictures online with private parts is an easy way to start. You then get to assess what has been stumbled upon/shown to them. You also keep the lines of communication open for the difference between art/nudity and pornographic material. And you don't vilify or make a big huge deal out of it.
Check out the books by Kristen Jensen (Good Pictures, Bad Pictures Jr) and Liz Walker (Not For Kids. These read-a-long books will really help both you and your child navigate this.
If you are still unsure, think about this scenario: you have a fire-pit or a BBQ in your garden. You are going to warn your three year old to keep away at safe distance or they may get hurt. You don't have to tell them the in's and out's of first degree burns. As with possible exposure to porn, you don't have to tell them more than they need to know at that age to keep them safe :)