Source : Webster's Revised Unabridged Dictionary (1913)

Teens \Teens\, n. pl. [See {Ten}.]
   The years of one's age having the termination -teen,
   beginning with thirteen and ending with nineteen; as, a girl
   in her teens.

Source : WordNet®

teens
     n 1: the time of life between the ages of 12 and 20
     2: all the numbers that end in -teen