Loss of eggs. Reproductive aging is simply the loss of eggs over time. At puberty one has ~400, 000 eggs and at menopause fewer than 100. The loss in general occurs steadily thru the years. Known things that accelerate the process are smoking, endometriosis and chemotherapy or radiation treatments. This is the hardest thing we have to treat in reproductive medicine.
It's unfair... Men make new sperm every 10 weeks, but women don't make new eggs. Therefore, like anything we put aside on a shelf (except, perhaps, for a good wine!), egg quality and quantity decline as a woman ages. This process begins when she is in her 20's and becomes exponential when she is in her late 30's. I hope this helps answer your question.