Adolescence
Adolescence (lat adolescere = (to) grow) is a transitional stage of human development that occurs between childhood and adulthood.  Adolescent humans go through puberty, the process of sexual maturation.  Teenagers (ages 13-19) are usually adolescent, though in some individuals, puberty may extend a few years beyond the teenage years, and in some individuals, puberty begins in the pre-teen years.

# Usage
In common usage around the world, "adolescent", "teenager", "teen", "youth", "young adult", "youngster", "young person" and "emerging adult" may be considered synonyms - although the term 'teenager' is an artifact of the English counting system, something which does not occur in all languages. The Oxford English Dictionary cites the first usage of the term to a Popular Science Monthly issue of April, 1941, "I never knew teen-agers could be so serious."
In sociology, adolescence is seen as a cultural phenomenon for the working world and therefore its end points are not easily tied to physical milestones. The time is identified with dramatic changes in the body, along with developments in a person's psychology and academic career. At the onset of adolescence (often referred to as 'puberty'), children usually complete elementary school and enter secondary education, such as middle school or high school. A person between early childhood and the teenage years is sometimes referred to as a pre-teen or tween.
As a transitional stage of human development, adolescence is the period in which a child matures into an adult. This transition involves biological (i.e. pubertal), social, and psychological changes, though the biological or physiological ones are the easiest to measure objectively.
The ages of adolescence vary by culture. The World Health Organization (WHO) defines adolescence as the period of life between 10 and 19 years of age. In contrast, in the United States, adolescence is generally considered to begin somewhere between ages 12 and 14, and end from 19 to 21. As distinct from the varied interpretations of who is considered an "adolescent", the word "teenager" is more easily defined: it describes a person who is thirteen to nineteen years of age.
During this period of life, most children go through the physical stages of puberty which often begins between the ages of nine and thirteen. (See Puberty below)
Most cultures regard people as becoming adults at various ages of the teenage years, often at the age of eighteen. (See Social and cultural below)

# Puberty
Puberty is the stage of the lifespan in which a child develops secondary sex characteristics (for example a deeper voice or larger adam's apple in boys, and development of breasts and hips in girls) as his or her hormonal balance shifts strongly towards an adult state. This is triggered by the pituitary gland, which secretes a surge of hormones, such as testosterone (boys) or estrogen (girls) into the blood stream and begins the rapid maturation of the gonads: the girl's ovaries and the boy's testicles. Some boys may develop Gynecomastia due an imbalance of sex hormones, tissue responsiveness or obesity. Put simply, puberty is the time when a childs body starts changing as to look more like an adult.
The onset of puberty in girls appears to be related to body fat percentage. In most Western countries, the average age of a girl's first menstrual period, or menarche, fell in a decreasing secular trend. Girls start going through puberty earlier than boys. The average age for girls to start puberty is 10-12 while the average age for boys to start puberty is 12-14.

# Psychology
Adolescent psychology is associated with the notable changes in the behavior also known as Mood swing. The characteristics of adolescents, cognitive, emotional and attitudinal changes take place during this period, which can be a cause of conflict on one hand and positive personality development on the other.
Due to the adolescents' experiencing various cognitive and physical changes, it is frequently notable that they start giving more importance to their friends, their peer group, and less to their parents/guardians, due to the aggregated influence of whom they might go on to indulge in activities not deemed as socially acceptable, although this may be more of a social phenomenon than a psychological one.
In the search for a unique social identity for themselves, adolescents are frequently found confused between the 'right' and 'wrong.' G. Stanley Hall denoted this period as one of "Storm and Stress" and, according to him, conflict at this developmental stage is normal and not unusual. Margaret Mead, on the other hand, attributed the behavior of adolescents to their culture and upbringing. However, Piaget, attributed this stage in development with greatly increased cognitive abilities; at this stage of life the individual's thoughts start taking more of an abstract form and the egocentric thoughts decrease, hence the individual is able to think and reason in a wider perspective.
Positive psychology is sometimes brought up when addressing adolescent psychology as well. This approach towards adolescents refers to providing them with motivation to become socially acceptable and notable individuals, since many adolescents find themselves bored, indecisive and/or unmotivated.
It should also be noted that adolescence is the stage of a psychological breakthrough in a person's life when the cognitive development is rapid and the thoughts, ideas and concepts developed at this period of life greatly influence the individual's future life, playing a major role in character and personality formation.

# Sexuality
Adolescent sexuality refers to sexual feelings, behavior and development in adolescents and is a stage of human sexuality. Sexuality and sexual desire usually begins to appear along with the onset of puberty. The expression of sexual desire among adolescents (or anyone, for that matter), might be influenced by family values and influences, the culture and religion they have grown up in social engineering, social control, taboos, and other kinds of social mores.
The risks of adolescent sexual activity is sometimes associated with: emotional distress (fear of abuse or exploitation), sexually transmitted diseases (including HIV/AIDS) and pregnancy through failure or non-use of birth control. In terms of sexual identity, sexual orientation among adolescents may vary greatly across the spectrum from heterosexuality and LGBT orientations to pansexuality and sexual fetishism.
According to anthropologist Racheal and psychologist Albert Bandura,  the turmoil found in adolescence in Western society has a cultural rather than a physical cause, and societies where young women engage in free sexual activity have had no such adolescent turmoil until more recently when truthful information about deadly STD's has been made publicly assessible. However many of these studies have been proven false. 
The age of consent to sexual activity varies widely between international jurisdictions, ranging from 12 to 21 years, although some governments, such as Canada's, are planning to raise the age to at least 16 in an effort to reduce the incidence of lethal STD's, pregnancy among teenage girls, and the sexual abuse and exploitation of younger teens.

# Culture
In commerce, this generation is seen as an important target. Mobile phones, contemporary popular music, movies, television programs, sports, video games and clothes are heavily marketed and often popular amongst adolescents.
In the past (and still in some cultures) there were ceremonies that celebrated adulthood, typically occurring during adolescence. Seijin shiki (literally "adult ceremony") is a Japanese example of this. Upanayanam is a coming of age ceremony for males in the Hindu world. In Judaism, 12 year old girls and 13-year-old boys become Bat or Bar Mitzvah, respectively, and often have a celebration to mark this coming of age. Among some denominations of Christianity, the rite or sacrament of Confirmation is received by adolescents and may be considered the time at which adolescents becomes members of the church in their own right. African boys also have a coming of age ceremony in which, upon reaching adolescence, the males state a promise to never do anything to shame their families or their village. This was also continued among African-American slaves in the early days of slavery before the practice was outlawed. In United States, girls will often have a "sweet sixteen" party to celebrate turning the aforementioned age, a tradition similar to the quinceanera in Latin culture.  In modern America, events such as getting your first driver's license, high school and later on college graduation and first career related job are thought of as being more significant markers in transition to adulthood.
Adolescents have also been an important factor in many movements for positive social change around the world. The popular history of adolescents participating in these movements may perhaps start with Joan of Arc, and extend to present times with popular youth activism, student activism, and other efforts to make youth voice heard.

# Legal issues, rights and privileges
Internationally, those over a certain age (often 18, though this varies) are legally considered to have reached the age of majority and are regarded as adults and are held to be responsible for their actions. People below this age are considered minors and are children. A person below the age of majority may gain adult rights through  legal emancipation. Teenagers may be rebellious because they want to have the same rights and freedoms as adults. As a result some of those teens may obtain counterfeit ID/licenses which allow them to partake of those privileges. On the other hand, many teens are in no particular hurry to abandon their younger years, and enjoy their childhood.
Those who are under the age of consent, or legal responsibility, may be considered too young to be held accountable for criminal action. This is called the defense of infancy. The age of criminal responsibility varies from 7 in India to 18 in Belgium. After reaching the initial age, there may be levels of responsibility dictated by age and type of offense, and crimes committed by minors may be tried in a juvenile court.
The legal working age in Western countries is usually 14 to 16, depending on the number of hours and type of employment. In the United Kingdom and Canada, for example, kids between 14 and 16 can work at certain types of light work with some restrictions to allow for schooling; while kids over 16 can work full-time (excluding night work). Many countries also specify a minimum school leaving age, ranging from 10 to 18, at which a person is legally allowed to leave compulsory education.
The age of consent to sexual activity varies widely between jurisdictions, ranging from 12 to 21 years, although 14 to 16 years is more usual. Sexual intercourse with a person below this age is treated as the crime of statutory rape. Some jurisdictions allow an exemption where both partners are close in age - for example, two 15 year olds. The age at which people are allowed to marry also varies, from 9 in Yemen to 22 for males and 20 for females in China. In Western countries, people are typically allowed to marry at 18, although they are sometimes allowed to marry at a younger age with parental or court consent. In developing countries, the legal marriageable age does not always correspond with the age at which people actually marry; for example, the legal age for marriage in Ethiopia is 18 for both males and females, but in rural areas most girls are married by age 16.
In most democratic countries, a citizen is eligible to vote at 18. For example, in the United States, the Twenty-sixth amendment decreased the voting age from 21 to 18. In a minority of countries, the voting age is 17 (for example, Indonesia) or 16 (for example, Brazil). By contrast, some countries have a minimum voting age of 21 (for example, Singapore) whereas the minimum age in Uzbekistan is 25. Age of candidacy is the minimum age at which a person can legally qualify to hold certain elected government offices. In most countries, a person must be 18 or over to stand for elected office, but some countries such as the United States and Italy have further restrictions depending on the type of office.
The sale of selected items such as cigarettes, alcohol, and videos with violent or pornographic content is also restricted by age in most countries. In the U.S, the minimum age to buy an R-rated movie, M-rated game or an album with a parental advisory label is 17 (in some states 18). 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. In the United States, teenagers are allowed to drive between 14-18 (each state sets its own minimum driving age of which a curfew may be imposed), in the US, adolescents 17 years of age can serve in the military.  The age at which teens are allowed to serve in the military is generally younger then the legal drinking age. In Europe it is more common for the driving age to be higher (usually 18) while the drinking age is lower than that of the US (usually 16 or 18). In Canada, the drinking age is 18 in some areas and 19 in other areas.  In Australia, universally the minimum drinking age is 18, unless a person is in a private residence or is under parental supervision in a licensed premises.  The driving age varies from state to state but the more common system is a graduated system of "L plates" (a learning license that requires supervision from a licensed driver) from age 16, "P plates" (probationary license) at 18 and finally a full license after 2 years on P plates i.e. for most people around the age of 21.
The legal gambling age also depends on the jurisdiction, although it is typically 18. You have to be 21 to play cards at a casino.
The minimum age for donating blood in the U.S is 17 although it may be 16 with parental permission in some states such as New York.
A number of social scientists, including anthropologist Margaret Mead and sociologist Mike Males, have noted the contradictory treatment of laws affecting adolescents in the United States. As Males has noted, the US Supreme Court has, "explicitly ruled that policy-makers may impose adult responsibilities and punishments on individual youths as if they were adults at the same time laws and policies abrogate adolescents rights en masse as if they were children."
The issue of youth activism affecting political, social, educational, and moral circumstances is of growing significance around the world. Youth-led organizations around the world have fought for social justice, the youth vote seeking to gain teenagers the right to vote, to secure more youth rights, and demanding better schools through student activism.
Since the advent of the Convention on the Rights of the Child in 1989 (children defined as under 18), almost every country (except the U.S. & Somalia) in the world has become voluntarily legally committed to advancing an anti-discriminatory stance towards young people of all ages. This is a legally binding document which secures youth participation throughout society while acting against unchecked child labor, child soldiers, child prostitution, and pornography.