I started even before 5.
As soon as my daughter started to ask why this, why that, you know that age, how tiresome it can be, I started with the germs, good touch, bad touch, sex, where do the babies come from etc.
She was about 3 at that time and she always told her grandfather to wash his hands after going to the wc, because of the bad germs there. Mind you, grandpa was a retired surgeon, so he did wash his hands anyway, he just loved my daughter pointing him to it and other "medical" problems.
Like the time she told her granny that I use Tampax, because I bleed every month, (grandma was a nurse) and whether she uses them too. Well granny says no, I am an old woman I don't need them any longer.
so as I pick up my daughter, she runs to me and says, hey mummy, don't worry, you won't bleed much longer. And I say huh?
Well, granny doesn't need Tampax any longer, because she's an old woman. So just a few months and you won't need them anymore!
Now, aren't kids great!!
Take care, Renee