Ah no answer yet, alright let's bring up the Timeaus. Please listen, that dialogue does create the Myth of Atlantis but I'm not going there. Atlantis isn't real, it's at best a metaphor like the cave. But what the dialogue does show is for 4th century BCE Athens they recognized their own Dark Age and loss of writing and acknowledge Egyptian Records far predate their own.

So Critias tells the story he heard from a 90 year old man as a boy passed down By another old man who was there in the days of Solon (Law Giver/creator of Athenian Democracy)

The story Critas tells is Solon went to Egypt and learned the true history of Athens and how they fought against.... Atlantis... Saving the Mediterranean from their slave empire from the west. Obvious historical nonsense but allegorical to the whole Persian threat from the East. But the important point is that Plato uses Egypt and their preserved language as the true marker of history, the what is written in the Egyptian script goes back to the first generation of man before Hesiod and Homer.

" O Solon, Solon, you Hellenes are never anything but children, and there is not an old man among you. Solon in return asked him what he meant. I mean to say, he replied, that in mind you are all young; there is no old opinion handed down among you by ancient tradition, nor any science which is hoary with age. And I will tell you why. There have been, and will be again, many destructions of mankind arising out of many causes; the greatest have been brought about by the agencies of fire and water, and other lesser ones by innumerable other causes. There is a story, which even you have preserved, that once upon a time Paethon, the son of Helios, having yoked the steeds in his father's chariot, because he was not able to drive them in the path of his father, burnt up all that was upon the earth, and was himself destroyed by a thunderbolt. Now this has the form of a myth, but really signifies a declination of the bodies moving in the heavens around the earth, and a great conflagration of things upon the earth, which recurs after long intervals; at such times those who live upon the mountains and in dry and lofty places are more liable to destruction than those who dwell by rivers or on the seashore."

So at the least Plato assumed Egypt was special, ancient and ridiculously old, older than any society he knew.

It's a very interesting question, deserving of much more learned answers than mine.

Ancient people didn't believe they were the first people. The Romans and Greeks always spoke of previous people living in their land, and sometimes they even sprang from the ground (that's what "autochtonous" literally means, "itself from the ground"). For the Greeks, it was the peoples of the ancient tales of Troy, which M. Finley has shown to be Dark Age accounts of Bronze Age traditions in one of the most enjoyable reads for any initiate into Ancient History, "The World of Odysseus". Finley mentions how Homeric heroes go to war on chariots but dismount before battle. Today we know the Bronze Age empires were built on the military might of an elite cast of charioteers, and that they were used in battle. The Greeks of "Homer's time", let's say (for the sake of convenience) didn't use chariots anymore, and didn't know how they'd be used in battle, but they knew, from oral tradition, that chariots had been important, hence the disconnect. It would be like someone from today hearing about walkmans or gramophones, and adding them into their 80's or mid-century fiction, but plugging the walkman into a computer to charge it, or having the gramophone run on AA batteries.

The Greeks even referenced (sometimes) the Pelasgians, the ones that supposedly were there before them (whatever "them" meant). Homer mentions them as just another tribe of Greeks in the Illiad, while Hesiod and others place them in Argos, Epirus and Thessaly (or somewhere in between), most of them mention the name with connotations of "ancient" or "old" or even sometimes also "different". Ovid seems to be aware of these connotations and straight up calls all Greeks who took part in the Trojan War as Pelasgi in Latin. Something could be lost in translation though, it's hard to tell. Not much is known about these Pelasgians after all.

As for the oldest of the oldest peoples, the Sumerians, we know they believed in a world before the Flood, which began approximately 30.000 years before Christ, when the "kingship descended from Heaven and Alulim (or Adapa, maybe an early version of the name Adam) was king in Eridu", as mentioned in the Sumerian King List. But we also know from the latest language studies that the Sumerian language, isolate and bizarre as it may be, might be in fact formed out of the fusion of two or more languages, one that was most likely there before (called Proto-Euphratean), and one which arrived later (presumably the closest one to Sumerian as we know it). So, while the Sumerians are the oldest people we have written records of, they themselves were aware of the unnumbered generations that came before, and culturally marked the limit to their knowledge in a generational barrier, their Flood myth, which wiped out most knowledge (and most people too), and before which very little could be known.

There's not much we can tell, due to the lack of earlier texts. Archaeological remains tell us a of lifestyles and objects, but not of stories and choices, or beliefs.