During adolescence, your teen's physiology changes dramatically. At 10, he or she may have been a cute and cuddly child, but by the age of 20, your teen is a full-sized adult, possibly taller than you, weighing more, and maybe passing for your younger sibling rather than your child. While the exact age at which your teen's body starts changing depends on many factors, such as heredity and nutrition, the average age for girls to start physiological changes is between 10 and 12 years, while boys start on the average between the ages of 11 and 13 years.
The hormones produced by endocrine glands stimulate body changes and cause secondary sex characteristics, with the ovaries in girls increasing their production of estrogen, and the testicles of boys increasing the production of testosterone. Hormones produced by the adrenal glands result in acne, sweating, body odor, and the growth of armpit and pubic hair. In girls, breast size increases, as well as height and hip width, the process of menstruation begins, and hair grows on the legs. In boys, the voices changes (cracks and "drops"), shoulders become broader, height increases, and hair grows on the face, under arms, and on legs.
Due to the process of rapid growth, your teenagers will probably require more sleep, and you may notice that they're more clumsy than usual as they have to get used to a larger body. If your teens develop early or later than their peers, they may be concerned. Boys who develop earlier are looked up to by their peers and end up with leadership positions. Adults might assume that the early-maturing teen is cognitively mature, which may not be true. Girls who develop early are sometimes pressured into dating older teenage boys before they're ready emotionally, and often suffer anxiety, depression or eating disorders.