Some younger children actually actively "request" a baby sibling and may order either a brother or sister. This may be a good time to discuss the fact that we don't actually choose exactly when we get pregnant or whether the baby will be a boy or a girl, or the perfect playmate for our existing kids. While it is a great idea to talk about the concept of having a sibling, questions like "Would you like a baby brother or sister?" might better be avoided. Why?
They may answer that no, they don't want a sibling, or want only a sibling of a specific gender, and then expect you to honor their input. These are, obviously, matters about only you and your partner decide. Older children who may be quite aware how babies are made may be a bit embarrassed if they know that you are trying for a baby. If you have an only child, they may also be concerned about no longer being the center of attention.
Giving your child plenty of time to get used to the idea of having a brother or sister may be good for some, but distracting for others. One more thing to keep in mind is that pregnancy doesn't always come easily, even for those who already have older children. You may like to hold off on talking about babies until you are actually pregnant. For that matter, plenty of parents wait until the first trimester of pregnancy has passed before informing their children.