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For the video game rating with a similar age group see ESRB
In psychology, the term early childhood is usually defined as the time period from the age of two until the age of six or seven years. There are three simultaneous development stages:
Physical growth and development 
In this phase there is a significant growth of the brain - in a two year old the brain is already at 75% the size of a full grown adult, and in a five year old, the brain is 90% of the adult's brain size. The growth of the brain is followed by a surge in cognitive abilities. Around the age of five, children start speaking properly and master their hand to eye coordination.
Cognitive growth and development 
Called the preoperational stage by Jean Piaget, this is the stage during which the child repeatedly asks "Why?", and is used to build relationships with the child. The child can't yet perform the abstract thinking operations. The child has to be able to see what is being talked about, because they do not understand the concepts of logic, betrayal, contemplation, etc. This means that they think literally: if a child is told that they have to go to bed because "night is falling", they will ask how can the night (literally) fall from the sky. They also see the human characteristics in every object, e.g. the table "is bad" if they accidentally hit it with their foot and it hurts. They also exhibit egocentrism; not to be confused with egoism; that being said, they do not comprehend that the other person has beliefs and the children at this age think that what they think, everybody thinks. There is also a matter of perceptive centration, which causes the children to primarily see what is visually most prominent on someone/something, e.g. if a man has long hair, the child will think he's a woman.
Social-emotional growth and development 
Children manifest fear of dark and monsters and around the age of three notice whether they are a boy or a girl and start acting that way. Boys are usually more aggressive, whilst girls are more caring. However, aggression is manifested in two different ways: boys are more physically aggressive, while the girls are more socially aggressive (name-calling and ignoring). In this stage the individual differences become more prominent.
Infants and toddlers experience life more holistically than any other age group. Social, emotional, cognitive, language, and physical lessons are not learned separately by very young children. Adults who are most helpful to young children interact in ways that understand that the child is learning from the whole experience, not just that part of the experience to which the adult gives attention..
The most information learned occurs between birth and the age of three, during this time humans develop more quickly and rapidly then they would at any other point in their life. Love, affection, encouragement and mental stimulation from the parents or guardians of these young children aid in development. At this time in life, the brain is growing rapidly and it is easier for information to be absorbed; parts of the brain can nearly double in a year. During this stage, children need vital nutrients and personal interaction for their brain to grow properly. Children's brains will expand and become more developed in these early years. Although adults play a huge part in early childhood development, the most important way children develop is interaction with other children.
Other definitions 
According to National Association for the Education of Young Children, early childhood spans the human life from birth to age 8.
|Stages of human development
Early childhood, Childhood