Imaginary friends are a common in childhood

... there is now clear evidence that imaginary friends are a common feature in childhood development. ...

Some children will talk to their friend about the problem, others will play with their imaginary friend, which takes their mind off the problem and the unhappy feelings disappear. We also know that some children who have endured traumatic life events may also draw on their imaginary companions for support.

The imaginary friends of older children and adolescents are a much more private affair. Often unknown to parents and others, although a best friend might know about their existence. Older children are aware that parents, friends and others may show disapproval.

As part of my research I interviewed school aged children aged five to eleven years and the parents of the younger children. All the children said that their imaginary friends were important and why they were special to them. I have concluded that imaginary friends are often a very positive feature in a child's life.

They provide fun, entertainment, adventures and games. They are often good, kind and helpful friends, good at listening and always available. Some imaginary friends are not always co-operative or friendly, this however seems to make them more real and interesting to the child and sometimes helps them to express their feelings when there has been a problem. ...

via BBC News - Today - My pretend friend.

... Imaginary friends are made often in childhood, sometimes in adolescence, and rarely in adulthood. They often function as tutelaries when played with by a child. They reveal, according to several theories of psychology, a child's anxieties, fears, goals and perceptions of the world through that child's conversations. They are, according to some children, physically indistinguishable from real people, while others say they see their imaginary friends only in their heads. ...

via Wikipedia