My DD just turned 7. She has extremely limited knowledge about puberty, and none about sex or reproduction. She know that babies are born vaginally or via c-section, that she'll grow hair, breasts and get a period as she gets older. That's about the extent of her knowledge.
I've always adopted the principle of answer any question she asks in an age appropriate way. She's never asked anything beyond the above.
At what age (if at all) do I stop waiting and start initiating the conversation?