my SIL told me about the mechanics of sex when I was 5, I'd asked about a picture in an encyclopaedia that was taken of a nearly full-term baby inside the womb - I initially asked what the rope was.

She sat down with me and explained the mechanics of sex and how a baby is formed and grows etc. She then taught me all about periods and the changes that puberty makes to the body, which was great and what I needed as I started my period about 3 1/2 years later and was wearing a bra in grade 5. She also fell pregnant when I was 7, so I got to watch the growth etc.

There was about 18 months between each of her children, so they all learnt about where babies come from early as well and by the time the youngest was born, the eldest (of 4) was quite knowledgeable.