Peer group

A peer group is a group of people who are equal in some way. Those in a peer group have the same status and are about the same age. They often interact with the group as a whole. Members of a peer group often have similar interests and backgrounds.
Developmental psychology.
Developmental psychologists, Lev Vygotsky, Jean Piaget, Erik Erikson, and Harry Stack Sullivan, argued that peer relationships are important in a person's development, and teach about equality, reciprocity, cooperation, and intimacy. Modern research also shows that social and emotional gains are indeed provided by peer interaction.
Judith Rich Harris, in "The Nurture Assumption," argues that an individual's peer group significantly influences their intellectual and personal development. Several long-term studies also claim that peer groups improve school work.