Just as the tiny fertilized egg cell from which you began divided again and again to become a baby, the trillions of cells now making up your body continue to divide as you grow. The more cells you have, the bigger you become. Some cells divide to replace worn out cells and others divide to increase the size and change the shape of your body as you mature. Hormones—chemicals that are produced by glands and circulate in your blood—help direct the growth of cells in your body during the process of growing up.
Usually people are fully grown by the time they reach the age of 20. By the time a person is 30, however, the rate at which body cells renew themselves begins to slow down, and signs of aging appear. As time goes on, certain body cells—like those of the brain and nerves—are not replaced when they wear out and die.