American Heritage offers this:
teen n. 1. a. The numbers 13 through 19. b. The 13th through 19th items
in a series or scale, as years of a century or degrees of temperature.
2. A teenager. --teen adj. Teenage.
teen n. Archaic. Misery; grief. [Middle English tene, from Old English
teona.]
Surely teenage can't be said to mean "the age of misery and grief?" :)
Be kind. Be of good cheer.
Dick Ford