I was just wondering why does menopause only happen in a women's 40's or 50's and not other ages?

Age of Menopause: The average age of menopause (cessation of menses for one year) is approximately 55 in the U.S. Some women will experience premature ovarian failure much younger, even in their twenties. Women who have a partial hysterectomy will often have earlier ovarian failure than they would have otherwise because of the blood supply to the ovaries being affected. The age is generally the same as her mom.