As women age, they experience a decline in reproductive performance leading to menopause (average 51 ± 8 years). This decline is tied to a reduction in the number of ovarian follicles present in the ovaries. The events that lead to to the decline - i.e. ovarian aging - are unknown. The variability of aging could include environmental factors, lifestyle habits and/or genetic factors.

Reproductive aging in women is a continuous process that begins prior to birth and extends through the menopausal transition. The primary mechanism behind this process is the depletion of the ovarian pool of non-growing follicles (NGFs). Compared with other major organ systems, the human female reproductive system ages to the point of failure at a relatively young age (average 51 ± 8 years) The wide age range at which natural menopause occurs suggests that there is considerable variation between women in the reproductive aging process. It is currently unclear whether this biological variation is due to differences in the initial endowment of NGFs, or alternatively, due to differences in the rate of NGF depletion."

Source : Human Reproduction, Volume 23, Issue 3, Pages 699–708

Also see : Menopauseplugin-autotooltip__plain plugin-autotooltip_bigMenopause

The decline in the number viable ovarian follicles, which leads to the menopause, is well documented (see Wikipedia), but, from an evolutionary perspective the reasons for its prevalence in humans are unknown.

Only a few other mammals* are known to exhibit menopause - including orcas, short-finned pilot whales and chimpanzees.