no rule here, as with most things with being a parent, it depends on your children, their maturity and there ability to understand all the responsibilities that go with sex. Had to do it with both my children and it took place at diffent ages of ages, one was farther than the other.
But for me 2 golden rules 1) you have to do it as a parent, its not optional, 2) better to be too early than too late with it