A) 5 years is a looooong time to be trying to conceive. Trying unsuccessfully to conceive is quite stressful.
B) You can't tell when you're 25 whether you're going to be in that lucky group. If having children is very important to you, those odds are not great.
C) Many people want multiple children. They can't afford to wait until it's taking 5 years to conceive each child.
There might be, there might not be. There will likely not be many years left where the woman is most easily able to conceive a child.