Adolescence: Crash Course Psychology #20. Erikson’s Psychosocial Development Theory. How old is an adolescent? Development Stages in Middle & Late Adolescence. Stage of Adolescence. Cognitive, Physical and Social and Emotional Development throughout Adolescence. Why Teenage Forge A Sense of Identity During Adolescence. How Teenagers Forge Their Identities. James Marcia's Adolescent Identity Development. References.