Menopause
Menopause typically occurs between 49 and 52 years of age. The majority of women have their last period between the ages of 48 and 55. The average age of the last period in the United States is 51 years, in the United Kingdom is 52 years, in Ireland is 50 years and in Australia is 51 years. In India and the Philippines, the median age of natural menopause is considerably earlier, at 44 years. The menopausal transition or perimenopause leading up to menopause usually lasts 7 years (sometimes as long as 14 years).
Dynamical systems theory
According to this model, stage transitions between age intervals represent self-organization processes at multiple levels (e.g., molecules, genes, cell, organ, organ system, organism, behavior, and environment). For example, at the stage transition from adolescence to young adulthood, and after reaching the critical point of 18 years of age (young adulthood), a peak in testosterone is observed in males and the period of optimal fertility begins in females. Similarly, at age 30 optimal fertility begins to decline in females, and at the stage transition from middle adulthood to older adulthood at 48 years, the average age of onset of menopause occurs.