According to Wikipedia  > psychology  Development psychology is the scientific study of how and why human beings change over the course of life. 

There are 5 stages in human development which involves a couple coming together to have sexual interaction, which results in a woman conceiving a baby which stay in her womb between 7-13 months (mainly 9months) to give birth to a child.

Other stages are:-
* Infancy
* Childhood stage
* Adolescence stage
* Adulthood stage
  
Infancy:- From birth -36 months(3 years) the child is referred to as an infant. Most time it spend in sleeping. It sleeps throughout the day and night.
Around this period a child is dependent on others on feeding, housing,  clothing or security et al by others.

Childhood stage. This is when a child move from infancy to a child. This is between 3-9 years old. At this stage a child becomes matured in language and presence of memory and imagination. 

The most important characteristics is where a child learns how to communicate. It's expected a child starts to interact  with its mother with others of the same age (peers) and relate with the others.

Adolescence: it's the period between puberty to adult good. Before now it used to be 11-15 years old but in the contemporary world it's between 9,11-13 year old..
At this stage the teen will decide what he wants to become in future the friends he keeps,  he decide his own future.

Adulthood stage:-  Before now adulthood used to be 18 and 20 years and above where the adult becomes matured old in physic, emotion and care and are expected to contribute  meaningfully to  existence around this period. It is expected that the individual in this category married or get married such an individual is an adult and expected to live a matured life. How ever in a contemporary world it starts from 15.

 * Should an adult age starts from the age of 18 or 20?
*  What are the challenges an adolescent or teen-ager face in today world?