How Many Extracurricular Activities Should a High School Student Participate In? Apart from the period between birth and two years old, the body undergoes more rapid development during adolescence than any other time in a person’s life. In high school, between the ages of 12 to 18, adolescents undergo a series of emotional and physical developments. As they grow toward adult independence, some of these changes include: A significant shift in their identities, interests, and subcultures A desire for more autonomy can affect family routines Testing the boundaries set by