A teenager, or teen, is a young person whose age falls within the range from 13–19. They are called teenagers because their age number ends with teen.Someone aged 18 or 19 is also considered a young adult. Usage by ordinary people varies, and also varies in different societies.Most societies traditionally had a formal ceremony to mark the change from childhood to adulthood.n the United Kingdom (UK); teenagers and non-teens are mixed in secondary school. Teenagers attending secondary school (high school in the US) generally graduate at the age of 17 or 18.