I've been doing potty familiarisation for a few months now. I've simply had the potty around so DD can get use to it, sits on it occasionally when we read a book. DD comes up and watches me go to the loo if she feels like it. She is quite interested and we wave goodbye to the wee wee together - lol.
My next stage is to try and get DD to identify when she is doing a poo. Some-times its obvious, other times its a 'stealth poo'. I want her to be able to tell me that she either wants to or is doing one. There is no hurry, we just take those opportunities when they present them self.
I'm hoping that we can start to catch DD doing a poo then sit her on the potty (clothes on). At some point when DD can tell me when she is doing one, I will start to take her nappy off - see how we go at catching a 'live poo' in the potty - lol.
I don't think there is any particular age to start. I think a lot depends on what approach you choose to take, your consistency with that approach and like any-thing with bubs - what they want to do.
DD is 3yr 8 months - DS is 6 months