'Teenager' is a common everyday noun describing anyone between the ages of 13 and 19. The word comes from the suffix 'teen' at the end of these numbers. 'Juvenile' is a technical term meaning 'not adult'. As a noun, it is often used for animals - for example, to distinguish between the appearance of an full-grown bird and an immature one. When applied to humans: As a technical term, is often used in a legal context. For example, while an eighteen-year-old offender might be tried in a normal court of law and be sent to a normal prison, a fifteen-year-old would be tried in a juvenile court and be sent to a correctional facility for juvenile offenders. Both are teenagers, but only the one under 18 is classed as a juvenile. As an adjective, it can also be used in a more everyday sense to mean childish e.g. 'His behaviour is really juvenile'.