At fourteen, first menstruation.
At seven, new teeth grow.
At twenty-one, the kidney qi js full. At twenty-eight, muscles and  bones are strong.
At thirty-five, her face becomes dry.
At forty-two, she begins to have white hair.
At forty-nine, menopause ends.

The kidney is the organ goveming fluids in the human body. It receives the vital essence from the other organs: heart, spleen, liver, and lungs. So when other organs are full, the kidney gives a vital essence. When a person is getting on in age, her internal organs deteriorate and her vital essence is drained. Learn more about the theory of Chinese medicine, please read Women Qi and Blood.