The end of adolescence and the beginning of adulthood varies by country and by function.
Furthermore, even within a single nation state or culture there can be different ages at which an individual is considered (chronologically and legally) mature enough for society to entrust them with certain privileges and responsibilities.
For example, researchers in neuroscience or bio-behavioral health might focus on pubertal changes in brain structure and its effects on cognition or social relations.
Sociologists interested in adolescence might focus on the acquisition of social roles (e.g., worker or romantic partner) and how this varies across cultures or social conditions.
Cognitive advances encompass both increment in knowledge and in the ability to think abstractly and to reason more effectively.
The study of adolescent development often involves interdisciplinary collaborations.
In studying adolescent development, adolescence can be defined biologically, as the physical transition marked by the onset of puberty and the termination of physical growth; cognitively, as changes in the ability to think abstractly and multi-dimensionally; or socially, as a period of preparation for adult roles.
Major pubertal and biological changes include changes to the sex organs, height, weight, and muscle mass, as well as major changes in brain structure and organization.
During puberty, bones become harder and more brittle.
Puberty occurs through a long process and begins with a surge in hormone production, which in turn causes a number of physical changes.
It is the stage of life characterized by the appearance and development of secondary sex characteristics (for example, a deeper voice and larger adam's apple in boys, and development of breasts and more curved and prominent hips in girls) and a strong shift in hormonal balance towards an adult state.
Males experience their growth spurt about two years later, on average, than females.
During their peak height velocity (the time of most rapid growth), adolescents grow at a growth rate nearly identical to that of a toddler—about 4 inches (10.3 cm) a year for males and 3.5 inches (9 cm) for females.