A teenager, or teen, is a young person whose age is between thirteen and nineteen (13-19). They are called teenagers because their age number ends in “teen”. Someone aged 18 or 19 is also considered a young adult. In the United Kingdom, a person aged 16-19 is often considered a young adult.

Teenagers go through a lot of physical and mental changes during this time. This is called adolescence or puberty.

In the United States, some teenagers go to middle school and some go to high school. In the United Kingdom (UK), teenagers and non-teens are mixed in secondary school. Teenagers attending secondary school generally leave at some point between 16 and 18.