Eggs age. Eggs are present from birth, and, for the most part, are never reproduced. That aging effect reduces the quality of eggs, and increases the chance of an error at cell division.
Female fertility. 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.