Why do they say fertility goes down rapidly in women after age 35?

Aging: The "why' questions in medicine are often unanswerable. The situation just is. In the case of female fertility, the ovaries and the eggs a female was born with begin to age and by aging they are less likely to work or provide the necessary support should implantation or pregnancy occur. A female does not produce new eggs. She has all of her eggs when she is born.