At that sort of age, a one-size fits all approach is totally inappropriate at any age. A child's capacity to understand information given, is very varied, thus the information must be specifically tailored to the understanding of that child. IMO, such an approach, is very much like taking a Power Press to crack a walnut.
Broadly, I am in favour of Rainchild's approach, the best time to give the information, is when the question is asked. It is relatively easy for the parent or guardian of a child, to establish the level of that child's understanding and give information appropriately.
This has the additional benefit, of building a rapport with the child, essential for more detailed discussions in future. Both my sister and myself, have adopted this approach with our respective children. As a result of this, the rapport we have with our children is significantly, closer than we had with our parents.