The World Health Organization (WHO) defines an adolescent as any person between ages 10 and 19. This age range falls within WHO’s definition of young people, which refers to individuals between ages 10 and 24.

While many individuals may refer to adolescents as teens, the adolescence definition according to World Health Organization is the period between the ages of 10 to 19. Some sources may set a broader range, and others believe adolescence should continue until the mid to late 20s when the brain finishes developing.

Age provides only a rough marker of adolescence, and scholars have not agreed upon a precise definition. Some definitions start as early as 10 and end as late as 25 or 26. The World Health Organization definition officially designates an adolescent as someone between the ages of 10 and 19.

The age ranges for these three stages may be debated, however. Experts often believe that adolescence begins with puberty, or around ages 10 to 14. By middle adolescence (ages 15 to 18), symptoms of puberty may subside, and teens might be closer to their adult height, weight, and size if they are in good health.

Teenagers who are between 13 and 17 years old are considered both children and teenagers in most countries. Teenagers who are 18 and 19 years old may be regarded as both teenagers and adults.