Adolescence (lat adolescere = (to) grow) is a transitional stage of physical and mental human development that occurs between childhood and adulthood. This transition involves biological (i.e. pubertal), social, and psychological changes, though the biological or physiological ones are the easiest to measure objectively. Historically, puberty has been heavily associated with teenagers and the onset of adolescent development.www.youthspecialties.com In recent years, however, the start of puberty has seen an increase in preadolescence and extension beyond the teenage years, making adolescence less simple to discern.

The end of adolescence and the beginning of adulthood varies by country as well as by function, as even within a single country there will be different ages at which an individual is considered mature enough to be entrusted with particular tasks, such as driving a vehicle, having sexual relations, serving in the armed forces, voting, or marrying.

Puberty

main: Puberty

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.Fact: date=September 2007 Girls start going through puberty earlier than boys, although there have been cases of boys having signs of puberty as early as the age of 9. The average age for girls to start puberty is 10-12 while the average age for boys to start puberty is 12-14.

Psychology

Main: Adolescent psychology

Adolescent psychology is associated with notable changes in mood sometimes known as mood swings. Cognitive, emotional and attitudinal changes which are characteristic of adolescence, often take place during this period, and this can be a cause of conflict on one hand and positive personality development on the other.

Because the adolescents are experiencing various strong cognitive and physical changes, for the first time in their lives they may start to view their friends, their peer group, as more important and influential than their parents/guardians. Because of peer pressure, they may sometimes indulge in activities not deemed socially acceptable, although this may be more of a social phenomenon than a psychological one. This overlap is addressed within the study of psychosociology.