Adolescence is a period of transition between childhood and adulthood.
The onset of adolescence has indeed changing with time: nowadays adolescence starts around 9 and 10 years of age and goes on about 18 or 20. Adolescence ca be described from different view points: from a physiological POW as an individual process, from a historical POW as a historical phenomenon and from a social POW as a social condition linked to different cultures. In the passage from childhood and adulthood there are several changes. From a physical POW girls start to menstruate and so become fertile the signs are: breast development, body hair, and more adult shape in Boys the voice breaks, facial and body hair appear as well as, involuntary erections. These physical changes influence heavily how adolescence relate to others and how they perceive them selves.
From a psychological POW adolescence can be defined as a process of differentiation from adults. Adolescence is a period of growing and changing and as all transitions and changes it is a process that involves personal turmoil.