The average age of menopause in the United States is around 51 years old, but the onset can widely vary. Premature menopause refers to menopause of onset at or before 40 years of age. This can occur because of a variety of causes, including surgery (i.e. bilateral oophorectomy, removal of ovaries), chemotherapy or pelvic radiation treatments for cancer, chromosomal or genetic defects, and spontaneous premature ovarian failure.