The Collaborative International Dictionary of English v.0.48:

Teenager \Teen"ag`er\ (t[=e]n"[=a]j`[~e]r), n.
   a person whose age is in the teens, i.e. one between the ages
   of 13 to 19 inclusive.
   [PJC]

WordNet (r) 3.0 (2006):

teenager
    n 1: a juvenile between the onset of puberty and maturity [syn:
         adolescent, stripling, teenager, teen]