A lot vs. Alot: 9 Grammatical Pitfalls
1670s (plural), "teen-age years of a person," formed from -teen taken as a separate word. As "decade of years comprising numbers ending in -teen," from 1889.
"teen-aged person," 1818 (but rare before 20c.), from -teen. As an adjective meaning "of or for teen-agers," from 1947.