that and also that as genetic abnormalities and diseases start to find cures, they will eventually die out of the genetic strains of humanity.
Cancer, and many other diseases caused by genetic issues will go away with in the next century or sooner, So as life spans start to increase because these underlying issues are gone, the ability to procreate at an older age will also come back into humans.
You must remember, that as a species humans DO NOT have a great genetic diversity.
Some great past calamity, more than likely Krakatoa, reduced the breeding population of humanity to a VERY VERY small number.
Only a few thousand (we arent sure exactly how many possibly about 20, 000) years ago.
Every human on this planet right now is related to those few survivors.
Due to such a low surviving breeding population, the diversity of Homo Sapiens and indeed many of Earth's native fauna, fell of DRAMATICALLY.
Humanity is JUST NOW reaching the levels of genetic diversity that we had before Krakatoa.
Genetic diversity leads to longer lives, healthier off spring, ect.
So as that diversity once again goes up, humans will live longer and breeding cycles will expand.
As we have longer lives, we will have more time to have children.
Toss in modern medicine and genetic elimination of deadly maladies and with in the next century, maybe sooner "Breeding " and "fertile" years will become a moot point.