Unknown, unfortunately

It is necessary to tell the children where the children come from and why sex is needed.
You also need to talk about contraception so that there are no children aged 14-15 years.
But the fact that sex is needed not only to have children, but also to get pleasure, can not be told before the age of 16. Even until 18.

The child will ask why you need contraception. Because if we don't know about sex for pleasure, then you don't want to have children - you don't have sex, why contraception? Not to mention the fact that under the influence of hormones children will guess themselves: tell not tell, and to the opposite sex draws.