Why does a woman's fertility decrease as she ages?

Eggs. All women are born with a finite number of eggs. As women go through life, there is a decrease in egg number. In addition, there is also a decrease in egg quality. Reduced egg quantity and egg quality are rsponsible for the reduced fertility as women go through life.
Egg number. A girl is born with every oocyte she'll ever have . Unlike men who produce new sperm regularly, females lose eggs from birth on; by ovulation, eggs getting 'old, ' and programmed cell death. Besides losing eggs each year, it is believed oocytes lost or used earlier in life are healthier or more fit for conception than the older ones. Thus both egg number and quality drops every year.