Immediately, as soon as the child has interest. Usually, children ask "where do the children come from?" That's right now and you need to tell.
How to tell?
First of all, without embarrassment, not to cause increased interest, because the forbidden fruit is sweet.
A small child does not have an attraction to the opposite sex and for him, writing and eating the same limbs as the arm or leg.
It's so simple at this level and we must tell without unnecessary details. But not about a stork.
Today it is not difficult to buy a book, preferably with pictures and prepare.
To lie or the whole truth
Only the truth, but not in all details, but as a child. If you turn up, when the fraud is opened, your explanation will be empty.