Adolescence is the transitional stage of development from child to adult. That time is identified with dramatic changes in body, psychology and academic career. In the onset of adolescence, children usually complete elementary school and enter secondary education, such as middle school or high school.
During this period of life, most children go through the physical stages of puberty, which often begin before a person has reached the age of 13. Most cultures regard people as becoming adults at various ages of the teenage years. Many cultures have a formal age of majority when adolescents formally become adults. In western cultures, this is usually demarked by a birthday, such as turning eighteen.
Puberty, occurring during adolescence, is the stage in the human lifespan when a child begins to develop adult secondary sex characteristics as their hormone balance shifts towards an adult state. Briefly, this is caused by the hypothalamus gland that secretes hormones into the blood stream which trigger growth in the gonads: the girl's ovaries and the boy's testicles. In female adolescents, the first menstrual period is called 'menarche'.
In developed countries, the age at puberty has reduced in the last century, primarily because of better nutrition. That change is particularly apparent in the countries that adopted western food. Girls tend to go through puberty earlier than boys.
The word 'preteen' describes a child a bit younger than a teenager: perhaps between the ages of about 8 and 12. The neologism 'tween' has the same meaning. This word comes from the age being between that of a child and a teenager. Preteens are increasingly a specifically targeted market segment by business, because they tend to maintain the preferences they develop at this age. Examples are personal devices such as cellular phones targeted to this segment.
A 'teenager' or 'teen' is a person whose age is a number ending in '-teen' in the English language: that is to say, someone from the age of thirteen to the age of nineteen. The word is of recent origin, only having appeared in the mid 20th century. In Western culture, a distinct youth culture has developed. This culture is often considered inferior to the mainstream culture, or in rebellion against it, and is thus often referred to as a subculture or counterculture. Teenagers in some cultures are often noted for their obsession with clothes, pop culture and gadgets. In commerce, this generation is seen as an important target, as many teenagers first develop spending habits with casual and part-time paid employment.
Psychology of adolescents
Maturity in body leads to an interest in sexual activities, sometimes leading to teenage pregnancy. Since they may not be emotionally mature enough or financially able to support children, the latter is usually considered problematic. At this age there is also a greater probability of drug abuse and alcohol abuse, or mental problems such as schizophrenia, eating disorders and depression. The emotional instability among some adolescents also sometimes causes youth crime.
Search for a unique identity is one of the problems that adolescents often face. They start to ask metaphysical questions about the meaning of life. At this age, role models such as sports players and rock stars are popular, and adolescents often express a desire to be like their chosen role model. For this reason, people who are considered role models are often heavily criticised for their behaviour, because of the perceived danger of adolescents copying them.
In many countries, those over a certain age (perhaps 18, though this varies) are legally considered responsible adults. Those who are under the age of legal responsibility may be considered too young to be held accountable for criminal action. This is called the defense of infancy. The right to vote in elections is given between the ages of 16 and 21 in many countries.
The sale of selected items such as cigarettes, alcohol, videos, and video games with sexual or violent content, is often prohibited based on age. Such age restrictions vary widely. In practice, it is common that young people engage in underage smoking or drinking, and in some cultures this is tolerated to a certain degree. Given the emotional immaturity of adolescents, many countries consider those under a certain age to be too young to engage in sexual intercourse and other sexual activities, even if they are physiologically capable. See age of consent. This issue has been dramatised in the 1962 movie Lolita. In many countries, sex with adolescents has become a social issue and is considered a serious sex crime.