Menopause...

The average onset of menopause is 51 years, but some women reach menopause at a younger age, especially if they have had cancer or another serious illness, and have undergone chemotherapy.

Premature Menopause

Premature menopause (or premature ovarian failure) is defined as last period ever occurring before the age of 40; it occurs in 1% of women. Causes of premature menopause include autoimmune disorders, thyroid disease, and diabetes mellitus. Premature menopause is diagnosed by measuring the levels of follicle stimulating hormone (FSH) and luteinizing hormone (LH); the levels of these hormones will be higher if menopause has occurred.

Menopause in Twins

Rates of premature menopause have been found to be significantly higher in fraternal and identical twins; approximately 5% of twins reach menopause before the age of 40. The reasons for this are not completely understood. Transplants of ovarian tissue between identical twins have been successful in restoring fertility.