[Magic.]

Puberty and sex are kinda different in my eyes, so learning about them is different too.
I would teach kids about puberty as they ask and be as honest as possible. When I was a kid and my mum bought razors she explained they weren't for dad like I thought, but "girls get hair too when they grow up". I don't think telling kids about puberty is bad at any to be honest, it's better they know about it and that it happens to everyone, rather than it happening and scaring them ...
As for sex, I think it should be age appropriate as and when they ask. Like I would never tell a child "a stork brings babies." But if a three year old asks where babies come from "mummys tummy" is a perfectly good answer. But when they're older and know more about relationships and puberty then they can be told more.