I wonder when the word "teenager" entered our vocabulary. "Teenage" in the
OED
means "tending"; they don't list "teenager," unless I've overlooked
something.
I don't have the supplements to the dictionary, though.  Anyway, I thought
"teenage" and "teenager" in our sense of the words originated in the 20th
century, although if the word was in use in the late 19th and first decade
of
the 20th century, Twain could certainly have used it.

Barbara Ladd