In his book "days and works" Hesiod speaks of four generations of man. Previous to his the "age of iron"
The golden generation of mortal man. The silver generation of man. The bronze generation. "The black death seized them" the cataclysmic even that ended the bronze age? The generation of hero men.
Are these hints at previous civilisations or epochs that have otherwise been forgotten or wiped out, and the proceeding generations are man starting from infancy after some unknown event(s)
Am I missing some point?
I think that you're looking for a historical origin to something that's primarily a literary trope. The world's going to hell and the general sense of cosmological decline that will lead to the world's destruction is somewhat common in mythological stories, and the idea that destruction came about due to sin (the silver age) is not unknown either. The heroic age allows for a sort of space to put mythological event in to justify why things don't seem to run like that anymore (note Book 5 of the Iliad for Homer doing the same sort of thing). You can hammer the square peg into the round hole here, but it seems to me like the literary theory satisfies things just as well.