In Yiddish, a nar is a fool, and narishkeit is “foolishness.” If you know Hebrew, though, you may do a double-take. Nar looks a lot like Hebrew na’ar, which means “boy, lad, youth” (and sometimes “servant” or “soldier”). The resemblance practically invites a joke – “so Hebrew is calling youths fools?” – but etymologically it’s just a coincidence. The Yiddish and the Hebrew aren’t related.
In Hebrew, na’ar shows up in three separate forms: a noun and two unrelated verbs. There is a verb meaning “to shake” or “to shake out,” and in one vivid biblical use it goes beyond shaking to something like “hurling” (G-d “hurls” the Egyptians into the sea). There is also a rarer verb meaning “to roar” or “growl,” which later Hebrew associates especially with the bray of a donkey. As for the noun na’ar (“youth”), two main theories are offered for its origin. One connects it to the “shake/throw” verb: the young are “that which is brought forth.” The other connects it to the “roar” verb, as an allusion to the roughness of the voice at the beginning of puberty.
Yiddish nar, by contrast, comes from German Narr, “fool.” The deeper origin of Narr is uncertain; one suggestion traces it to Latin naris, “nose,” via sneering and mocking. In any case, German had the word long before modern Yiddish was standardizing, so the borrowing direction is almost certainly from German to Yiddish.
And anyway, if we’re being honest, narishkeit isn’t reserved for any particular age.