I've got to say, my DC were 8 when I first bought a few books on what happens when you grow up. My idea was that they'd read the book & ask any questions.
When eldest DC was 3, my SIL was BF in the weirdest possible way. He said oh that's Mummy milk, why can't I have Mummy milk anymore. We said you eat proper food now, as you have teeth.
We didn't think the need to go into graphic detail about anything, no questions were asked, which is why I ended up getting these books. Eldest DC was so proud telling everyone Mum has ovum and Dad has sperm, then you get a baby.
The books got passed down, then there was another book about teenagers, feelings, that kind of stuff. So about 12 we introduced that one. As DC got older we would ask about school romances.
My parents were liberal as this Scandinavian policy was about being open. My Dad would happily talk about contraception and other stuff. I just get the idea eldest DC might be mortified if we did that with them, we do say if you think you might do it, then you always need a condom to protect against diseases. With our youngest DC we'd have to talk about the pill & contraception, but that's a long way off.
My Dad even got into a debate at a party, I was heavily pregnant, these girls were 14/15 and the idea of a baby was heaven to them. With some strange idea you just push baby round town, life is bliss, there's no negative side at all.
He said to their Mum about Amsterdam and Scandinavian countries who are open about sex, the teenage pregnancy rates are low. The Mum though felt that it shouldn't be spoke about, it got into a bit of a debate. Both debating adults had been drinking so it made for some different entertainment.
I think really foundations should be laid down by 12, with supportive quick chats from parents / caregivers.
At school you got told about periods when you were 9/10, got a free goody bag. Sex education was pretty poor, it wasn't really addressed till we were 14. By that time we had a couple of girls with babies in our year.
There is the curiosity of I've got this, you've got that, how does this work then. A totally different debate I guess as to whether you should support experimenting or go with the make it special route.