Weird historical linguistical thought

I've read in a bunch of works on the history of childhood that the concept of "teenagerhood" as a separate period of life is a twentieth-century concept (Depression era, specifically: post WWI.)

Something just hit me out of the blue about a minute and a half ago: if that is true, why do both Biblical Hebrew and Classical Latin have words for "teenager"? (na'ar/na'arah for Hebrew, iuvenis for Latin)