Before age 11 or 12 children think in terms of concrete objects and groups of objects. By age 16 most adolescents have gone from the simple way of thinking to complex forms of reasoning. They learn to approach a problem systematically. Moral issues become more complex because they understand that two sound rules or principles might conflict. They will struggle with a question about whether someone should report a friend for breaking a rule.
Teens also come to realize that what exists is only one of many possibilities.
This is important in facing many choices as they move into adulthood and choose career directions, educational paths and mates. Thus, teens need time alone to think about the many possibilities. Because of their physical and mental growth, adolescents are no longer treated like children. The expectations adults and peers have of them change and their behavior changes. Thus the social world in which they live changes in important ways.