The moment they show any interest at all. For my 9 year old he wanted to know where babies came from when I was pregnant with his siblings. He got a very generic answer when he was 2 asking this because he had no concept of what he was asking. However when he was 7 asking this he was given a slightly more in depth answer. Earlier this year he was given "THE TALK" and he is 9.
It is ignorant to believe waiting longer will somehow prevent sex from happening. So many people think that by talking about it, that we would somehow encourage it or make it happen, and to that I will say I have talked about winning the lotto plenty and have yet to win.
Open communication lines with children is one of the most valuable things we can do as parents. So for me, none of my children will ever be sheltered from facts. It is my job to inform, teach, and if all goes well prepare them for adult hood in every way possible.