How does advancing age cause infertility?

Egg # and quality: Although there now appear to be some stem cells in the ovaries, as women get older there are fewer viable eggs left in the ovaries (diminished ovarian reserve). More of the eggs have genetic abnormalities as a woman gets older, which can make it harder to conceive, or increase the chance of miscarriage or abnormal pregnancy. Problems like fibroids or polyps may affect the uterus at any age as well.

Often egg related: A woman is born with all the eggs she will ever have and as time goes by those eggs get used up and there is an ever-declining number of eggs available. Not only does the number decline, but also those eggs remaining as a woman gets older are often those of lesser quality.