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