Teenager

google

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early 20th century (originally in the US): from teenage + -er1.

wiktionary

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teenage +‎  -er

etymonline

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teenager (n.)

also teen ager, teen-ager; 1922, derived noun from teenage (q.v.). The earlier word for this was teener, attested in American English from 1894, and teen had been used as a noun to mean "teen-aged person" in 1818, though this was not common before 20c.