It is important for social scientists to study mental illness during adolescence and young adulthood. Adolescence is considered to be the life stage that generally represents the teenage years. According to sociologists, adulthood is when adult social roles (e.g., spouse, parent, and full-time employee) have been achieved. We explore the definitions of adolescence, the transition to adulthood, emerging adulthood, and young adulthood. Research suggests that the timing of the transition to adulthood has intriguing mental health consequences. Although sociologists emphasize that the transition to adulthood does not have a definitive length, this early life stage is known as pivotal for the development of symptoms of psychological distress and the onset of mental illness. In particular, rates of distress, depression, and alcohol abuse/dependence peak during young adulthood. We also discuss the research evidence and some gaps in our knowledge about the effects of different social statuses – socioeconomic status, gender, race/ethnicity, marital status, and sexual orientation – on mental health among adolescents and young adults. Finally, we argue that public policies and interventions need to target the prevention of mental illness by focusing on the social risk factors in adolescence and young adulthood. Keywords: depression; life course sociology; sociology of aging; sociology of mental health; stress