Teenager to Teen

Does anyone know when the word "teenager" became simply "teen"? I still use the former, which indicates that I'm not the latter! I know the word "teenager" itself was coined in the 1920's by a sociologist.

I first noticed the transition about two years ago when I heard a radio commercial in which an actor putting on an "old guy" voice used the phrase "when I was a teen". No old guy I've yet met would say that!

Thanks...

They might know the answer to that at askoxford.com (I surely don't).

I would fairly tentatively suggest that in US the word "teen" has always been used whereas in UK "teenager" has been and still is the consistent usage