Teenager is a numeric term for a person from the ages of 13 to 19 years. People aged 10 to 12 years old are placed in the category of preteen, which was coined to recognize ages 10 to 12 as part of the same decade as 13-19 but linguistically separate due to the absence of -teen. The shortened form of teenager is teen, which is the only recognized form of the suffix used as a noun or adjective.
Although teenager is primarily a numeric age term, it is commonly used to describe a person in the process of transitioning from childhood to adulthood (primarily 13 to 17 years old) while experiencing the physical changes of puberty, and is conflated with adolescent. However, the developmental changes of adolescence, such as puberty (particularly in females) and the transition from primary to secondary school, typically begin around the preteens, while cognitive and physical maturation continue into the 20s. Thus, the teenage years only provide a very approximate age range of human adolescence.