I don't think you can start giving information too early - others may disagree with me on this. But I beleive the younger they are the more time they have to 'understand' the information given to them and get more information about things they aren't sure about. I do think you can leave it too late. If they are in high school they should know at least some things by now - if you don't tell them I can assure you they are getting the info from friends - and is likely to be quite inaccurate.

My parents didn't ever have a big sex talk as such. Rather from the time I was young (like a toddler) we had books in the house (where did I come from, etc) that we were free to read and which my mum would read to us every now and then over the years. Of course if we did have questions we were very free to ask. It always amazed me how littl my friends new about reproduction and their own bodies in high school, even as old as year 11 and 12.