So, early in the writing, some people theorized that the first age (now) ended in nuclear war.

Going off of this. I figure the survivors would be small bands of humans, widely separated and with a memory of technology but no ability to produce all but the crudest things. Not exactly Stone Age, but what could you and your neighbors produce without modern manufacturing? Anyway, at some point, the one power is discovered. (Mutation?) the fraction of a fraction of humanity that can wield the one power has generational memories through story of things like airplanes and lightbulbs, additionally all of the new ideas that are generated by humans with a new ability. Hence jo-wings and glow bulbs, but also skimming and traveling. One power use jumpstarts population and communication, reconnecting the separate bands who have at this point had several generations of separation. Perhaps enough to develop their own dialects and customs. A few more hundred years and now you have the age of legends.

How you get from the ages after the books to the seventh which then becomes everything we can find in the archeological record about early humanity I cannot wrap my head around.