how old do you think someone should be before they have a baby?
I think there need to be a few things in place: preferably a long lasting, stable partnership, a state of financial comfort, owning a home, careers well in place, etc.
The age of child bearing is getting later and later; many families do not start until the parents are in their early thirties. Your ovaries will still be there in ten years, but the potential and opportunities you have now will not be.
You have to make sure you can be completely selfless; once you consider having a baby, you are not the most important person in your life anymore.
I think the earliest a person should plan for a baby is their late twenties.