If we count statements made about previous generations, these go back to some of the oldest literature. There’s a speech like this in The Iliad, set in the mouth of an ancient character of the epic, about how much greater an earlier generation was and young men should be more respectful to their elders. You could read this as Homer saying that old men have always felt that way, or that the generations have been in decline since time immemorial.
I am older than either of you; therefore be guided by me. Moreover I have been the familiar friend of men even greater than you are, and they did not disregard my counsels. Never again can I behold such men as Pirithous and Dryas shepherd of his people, or as Caeneus, Exadius, godlike Polyphemus, and Theseus son of Aegeus, peer of the immortals. These were the mightiest men ever born upon this earth: mightiest were they, and when they fought the fiercest tribes of mountain savages they utterly overthrew them. I came from distant Pylos, and went about among them, for they would have me come, and I fought as it was in me to do. Not a man now living could withstand them, but they heard my words, and were persuaded by them. So be it also with yourselves, for this is the more excellent way.
And neither Achilles nor Agamemnon does, leading to disaster.
There are several condemnations of entire generations in the Hebrew Bible, although they are difficult to date. The history running from Deuteronomy to II Kings (which most scholars date between the reign of Josiah in the late seventh century BCE and the Babylonian Exile in the sixth century BCE) sees ancient Jewish history as a cycle of successive generations becoming debauched and their children repenting, as in the second chapter of Judges:
After that whole generation had been gathered to their ancestors, another generation grew up who knew neither the Lord nor what he had done for Israel. Then the Israelites did evil in the eyes of the Lord [...] In his anger against Israel the Lord gave them into the hands of raiders who plundered them. He sold them into the hands of their enemies all around, whom they were no longer able to resist. Whenever Israel went out to fight, the hand of the Lord was against them to defeat them, just as he had sworn to them. They were in great distress.
Then the Lord raised up judges, who saved them out of the hands of these raiders. Yet they would not listen to their judges but prostituted themselves to other gods and worshiped them. They quickly turned from the ways of their ancestors, who had been obedient to the Lord’s commands. Whenever the Lord raised up a judge for them, he was with the judge and saved them out of the hands of their enemies as long as the judge lived; for the Lord relented because of their groaning under those who oppressed and afflicted them. But when the judge died, the people returned to ways even more corrupt than those of their ancestors, following other gods and serving and worshiping them. They refused to give up their evil practices and stubborn ways.
Several of the Hebrew prophets inveigh against their contemporaries. One word for those diatribes is still “Jeremiads,” after the prophet from the seventh century BCE. For example, Jeremiah 16:
It is because your ancestors forsook me, [...] and did not keep my law. But you have behaved more wickedly than your ancestors. See how all of you are following the stubbornness of your evil hearts instead of obeying me. So I will throw you out of this land into a land neither you nor your ancestors have known, and there you will serve other gods day and night, for I will show you no favor.
Jeremiah, though, does not think it is only the young who are less pious and moral than their parents, but all living Israelites.
Job does this on a more personal level, in chapters 29 and 30, but is more difficult to date.
Oh, for the days when I was in my prime, when God’s intimate friendship blessed my house, when the Almighty was still with me [....]
But now they mock me, men younger than I, whose fathers I would have disdained to put with my sheep dogs. [...]
And now those young men mock me in song; I have become a byword among them. They detest me and keep their distance; they do not hesitate to spit in my face.