According to The World Health Organization, adolescence is “the period in human growth and development that occurs after childhood and before adulthood from ages 10 to 19.” Similar to the changes experienced during infancy, individuals undergo a critical transition characterized by great biological, physiological, and emotional changes that occur rapidly. 
These biological changes help determine lifestyle patterns. Educational and social changes occur, affecting overall physical and psychosocial well-being and behavior. The measurement of mental well-being has many facets including self-esteem, life satisfaction, happiness, mastery, a feeling of self control, self-purpose, and a sense of belonging and support. 
The Webster’s New Collegiate Dictionary defines adolescence as the process of growing up, maturing. The word finds its root in the Latin word alescere, meaning to grow. 
Stage of life
Adolescence is arguably the most influential stage in life. The choices made and behaviors exhibited during this period largely help form an individual’s identity. There is no particular age when a person enters adolescence. Different countries have differing conceptions about what marks the age of adolescence. Voting rights, the ability to have a license, and the age of reason, which permits sexual conduct, vary among countries. This demonstrates the incalculable nature of adolescence. In the past adolescence had been marked in large part by the onset of puberty, but because the inception of puberty is different for everyone, and usually a long process, it is difficult to identify a particular age.
Many of the changes adolescents experience are not just physical but also very psychological. Teens become more independent, challenging authoritative figures, and rebelling. Many reasons contribute as to why both physical and mental changes transpire.
Development of sex drives leads to attraction which may result in conflicts between an adolescent individual and his or her parents, teachers, and other authority figures. Epistemologist and logician Piaget came up with four stages of cognitive development. The cognitive theory suggests that teens shift from the ‘concrete operational stage’ to the ‘formal operational stage.’ This is marked by the ability to problem-solve, create make-believe situations, hypothetical situations. It is during this time that teens acquire new ideas of abstract reasoning, importantly those related to moral judgments. Teens are trying to ultimately create a positive identity. Failure to do so can result in identity/role confusion and lead to delinquent behavior, behavioral maladjustment, promiscuous acts, and even criminal activity. 
The biological determinants of adolescence are fairly universal.  However, the duration and defining characteristics of this period may vary across time, cultures, and socioeconomic situations. This period has seen many changes over the past century namely the earlier onset of puberty, later age of marriage, urbanization, global communication, and changing sexual attitudes and behaviour. 
Sarah-Jayne Blakemore: The mysterious workings of the adolescent brain
Sarah-Jayne Blakemore suggests in her TedTalk video a different approach to viewing adolescence. Originally it was thought the vast majority of brain development occurs in the first fifteen years of an individual’s life. However, this theory has been challenged and debunked in the last decade. The way we think about brain development has changed. Brain development continues to develop through the late teen years into the early twenties. The pre-frontal cortex changes vastly during these years. Decision making, planning, inhibiting inappropriate behavior, self awareness are all features of this cortex. After adolescence, decline in gray matter function occurs, making it a salient feature of adolescence. During adolescent years, reading and interpreting nonverbal cues comes easier to individuals. Adolescents use a different approach to evaluate social circumstances. This video compares the prefrontal cortex in adolescents to that of adults, to show us how typically “teenage” behavior is caused by the growing and developing brain. 
Meeting Their Needs
Young adolescents need a nurturing, secure environment at school and at home, with comforting and reassuring guidance from adults. If preventative measures aren’t taken, problems can occur. There may be significant economic and social consequences such as increased dropout rates, teen pregnancy and motherhood, substance use and abuse, and life-long violent behaviors. A multitude of girls of this age group lose their self-esteem, struggling with issues of self-confidence. As young as twelve, girls require opportunities for leadership that can build confidence and independence. 
Larry Steinberg in his book Age of Opportunity: Lessons from the New Science of Adolescence outlines areas of improvement for educational, social and familial relationships, where adolescents are at risk of following societal norms. He questions the way societies raise its young, pointing to violence, unwanted pregnancies, STDs, abortion, binge drinking, drug use, obesity, and depression. 
One conception of juvenile delinquency believed by some, is that a large majority of assaults, violent and sexual, are committed by males. A popular and widely accepted reason for this is that many of these young males may be trying to display their masculinity and thus more likely to offend. Performing these dangerous, tough, and often aggressive activities may be a means of displaying their masculinity. Acting out these ideas will generally make males more willing to do these antisocial and criminal acts in an effort to be accepted by their friends and peers. For young people being accepted isvery important, and at times people may ‘flock’ to the wrong groups of people and be forced to do questionable activities. Another idea for why males are more likely to offend can be a biological reason. Males may just be more biologically aggressive. It should come as no surprise that adolescents with past trouble with the law viewed law enforcement worse than those with little to no prior involvement. Even though correlates of delinquency are on a whole similar between the two genders, social control can be used to explain the gap in delinquency between boys and girls. Analysis found that a range of measures of social controls connected with the family and school are related with youth delinquency. However, there are many other factors which explain the differences - for example, ethnicity, age, and leisure-time activities.