So you've run across a trope or story element that seems unspeakably old. It simply drips with antiquity and grandeur, so you assume that it must have been around since the first caveman. But then you research it a little more, and discover...that it was invented by a Los Angeles advertising executive in 1989?
Congratulations: You've learned that some things are Newer Than They Think — a relatively recent invention that people tend to assume has much deeper roots in history and popular culture than it actually does — or the roots are considerably further from the end result than you realize. It usually arises from the myth being tied into an older myth; or the assumption that because the mythology is old, it hasn't been changed. Sometimes the trope really is as old as they think, but it's only become popular within recent historical memory.
It may also tie into Our Monsters Are Different, as many "standard rules" seem like they should come from folklore and legends, but really come from more recent media. The development of fantasy RPGs has been a major mover in both tropes, as RPG creators have freely raided, adapted and bastardized from folklore and literature in order to fill their manuals, scenario books and bestiaries.
Some neologisms can be mistaken for being very old as well.
Compare Lost in Imitation (well-known elements of a story are a lot more recent than the story itself), The Newest Ones in the Book. Convincingly well-done Retraux is a common factor in this trope. See also Dead Unicorn Trope, where a trope that is thought of as old and Cliché wasn't actually present in the original work.
Contrast Older Than They Think, which goes the other way.
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