Adolescence

Adolescence is a transitional stage of human development that occurs between childhood and adulthood.
Usage
In common usage around the world, "adolescent", "teenager", "teen", "youth", "youngster", "young person" and "Emerging Adult" may be considered synonyms - 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.[1] 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. It is most commonly referred to people who attend high school and undergraduate college. [1]
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.
About the Major Developmental Changes
The average age for girls to start puberty is 10-12 while the average age for boys to start puberty is 12-14.
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 oestrogen (girls) into the blood stream The onset of puberty in girls appears to be related to body fat percentage.Girls start going through puberty earlier than boys.

Physical
The process of rapid physical changes in adolescence is called puberty. It starts gradually, from around eleven years for girls and thirteen for boys. So, your children may hit puberty earlier than you did. The hormone changes responsible actually begin some years earlier and may produce periods of moodiness and restlessness. Girls start these changes before boys and will, for the first three or four years, appear to be maturing much faster. After this, boys catch up.
These changes include:
For girls: menstrual periods, growth of under-arm, body and pubic hair.
For boys: voice breaks (becomes deeper), growth of body and pubic hair, facial hair, erections and wet dreams. For both: Rapid physical growth.
By the age of 17, they'll be young men and women who may be bigger than their parents and capable of having children themselves. In spite of this, they often still need support from you.
It is not surprising that, with the speed of these changes, some adolescents become very concerned about their appearance. They may need a lot of reassurance, especially if they are not growing or maturing as quickly as their friends.
They and their parents may worry less if they remember that there's a lot of difference in the ages at which these changes occur.
Growth and development uses a lot of energy, and this may be why teenagers often seem to need so much sleep. Their getting-up late may be irritating, but it may well not be just laziness.
Thinking changes: At the same time physical changes are taking place, early adolescents are beginning to develop new ways of thinking. They’re no longer caught in the "here-and-now" of childhood; they’re beginning to develop higher-level thinking skills that allow them to think about ideas, anticipate and begin to see a personal future. They may begin to question old beliefs and explore new ones, as well as to criticize the adults in their lives.
Teenagers are more selfish than adults because they use a different part of their brain to make decisions compared to adults, new research suggests.
Previous work has shown that when children reach puberty, there is an increase in connections between nerves in the brain. This occurs particularly in the area involved in decision-making and awareness of other people’s feelings, called the "mentalising network".

Social changes and Behaviour Problems
Social changes occur during early adolescence for many reasons. They are brought about because of the body changes kids are going through and the new thinking that kids are beginning to develop. The freedom and responsibility granted to kids by parents and society also contribute to social changes, because early adolescents are allowed to do more things and go more places. Peers become increasingly important during these years, a process that helps kids to gradually "wean" themselves from parents and become independent. Early adolescents also move from primarily having friends of the same gender to having friends of the opposite gender, marking the beginning of exploring and learning about male-female relationships. They make close relationships outside the family, with friends of their own age. Relationships within the family also change. Parents become less important in their children's eyes as their life outside the family develops.
Teenagers and their parents complain about each other's behaviour. Parents often feel they have lost any sort of control or influence over their child. Adolescents want their parents to be clear and consistent about rules and boundaries, but at the same time may resent any restrictions on their growing freedom and ability to decide for themselves.
If disagreements are common and normal, when should you worry? Experience suggests that children are at greater risk of getting into trouble if their parents don't know where they are. So, try to make sure that you know where they are going and what they are up to. If you really don't know, you need to find out.

Adolescent psychology
With many things changing both inside and outside of the early adolescent, life can seem pretty topsy-turvy at times
Adolescent psychology is associated with the notable changes in the behavior and 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.
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.[3] 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.[5]
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[6] 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.
Psychological and emotional problems
Emotional changes: It’s not surprising that amidst these all of these other changes, early adolescents sometimes feel unsure of themselves and need the support of adults.. If early adolescents act like children one day and adults the next, it’s because that’s exactly how they’re feeling!

Emotional Problems
Over-eating, excessive sleepiness and a persistent over-concern may be signs of emotional distress.
Anxiety may produce phobias and panic attacks. •
At some time, 4 out of 10 adolescents have felt so miserable that they have cried and have wanted to get away from everyone and everything.
During their adolescence, more than 1 in 5 teenagers think so little of themselves that life does not seem worth living. In spite of these powerful feelings, depression may not be obvious to other people.