Adolescence is the stage of development between childhood and adulthood; it generally refers to a period from 12 or 13 years to 19 or 21 years. Although its onset is often balanced with the onset of puberty, adolescence is characterized by psychological and social stages as well as biological changes. Adolescence can be prolonged, brief, or virtually nonexistent, depending on the type of culture in which it occurs. In simple societies, for example, the transition from childhood to adulthood tends to occur relatively quickly and is marked by traditionally prescribed rites of passage. In contrast, in American and European societies, the transition period for young people has continued to lengthen over the past 100 years, giving rise to an adolescent subculture. As a result of this prolonged transition stage, a variety of issues and concerns specifically associated with this age group have developed. Psychologists distinguish four areas that particularly affect adolescent behavior and development: physiological change and growth; cognitive or mental development; identity or personality formation; and parent-adolescent relationships. Physiological Change: Between the ages of 9 and 15, almost all young people experience a rapid series of physiological changes, known as the adolescent growth spurt. These hormonal changes include an acceleration in the body's growth rate; the development of pubic hair; the appearance of axillary or armpit hair about two years later. There are changes in the structure and functioning of the reproductive organs; mammary glands in girls; and the development of sweat glands, which often leads to an acne breakout. In both sexes, these physiological changes occur at different times. This time of change can be very stressful for a preteen. Because at this stage of life, appearance is very important. A teenager who develops very early or extremely late may be very ridiculed by his peers. However, the time at which a girl goes through this stage and the time at which a man goes through it are different. Girls typically begin their growth spurt shortly after age 10. They tend to peak around age 12 and end at age 14. occurs almost two years later in boys. Boys therefore go through a difficult period where girls are taller and heavier than them. This delicate period occurs between ten and a half and thirteen years. Time isn't the only difference in the pubescent period between boys and girls. In girls, breast enlargement is usually the first physical sign.