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.

There is this from Ecclesiastes:

Say not, “Why were the former days better than these?” For it is not from wisdom that you ask this.

So although he is admonishing not to make this statement, we do see that such sentiments existed, from which he steers us away.

Ancient

I have not studied the originals of these sources, but I was able to find this discussion attributing the following text to Plato's Republic (380 BC):

The quote may have come from Plato's Republic Book 4, where Socrates is quoted saying the following regarding things that he thinks have been neglected: "I mean such things as these: ? when the young are to be silent before their elders; how they are to show respect to them by standing and making them sit; what honour is due to parents; what garments or shoes are to be worn; the mode of dressing the hair; deportment and manners in general. You would agree with me? ? Yes."

In addition, this discussion mentioned the play Clouds by Aristophanes (423 BC), where a speech has the following text:

. . .

A boy must hold his tongue among his elders.

. . .

Greed was abhorred, it was taboo to snatch
Radish tops, aniseed, or parsley before your elders,
Or to nibble kickshaws and giggle and twine one's feet.

. . .

So, you shall learn to hate the Agora,
And shun the baths and feel ashamed of smut;

. . .

And to get up and give your seat to your elders,
And not to behave towards your parents rudely

. . .

The play is a comedy, but not in the modern sense of the word. I am not sure if it is meant as satire or not.

This article quotes Horace (Book III of Odes, 20 BC):

Our sires' age was worse than our grandsires'. We, their sons, are more worthless than they; so in our turn we shall give the world a progeny yet more corrupt.

Modern (verifiable)

The above article also has links to downloadable primary sources. The oldest of them is from The Wise-man's Forecasts Against the Evil Time from 1624:

Youth were never more sawcie, yea never more savagely saucie . . . the ancient are scorned, the honourable are contemned, the magistrate is not dreaded.

So if the ancient sources don't sit right by you, you can at least sleep soundly knowing that old people have considered us young'ins "savagely saucy" for 400 years.

You have great predecessors in your question

• You are not the first to notice the habit of Humans to complain about better times from the past being gone. Horace, in Ars Poetica, already called one of these nostalgic people "laudator temporis acti se puero".

• A useful source might be the book from French scholar Lucien Jerphagnon, C'était mieux avant, that collects and comments exactly the kind of quotes you are looking for. Alas, I cannot find extracts from the book online. If you can read French, it might be worth purchasing it.

Specific quote about children not listening to their parents

~2225 BCE, Akkad

This answer by @00prometheus from Skeptics.SE traces an article describing an inscription attributed to King Naram Sin of Chaldea, 3800 years B.C (much older than anything mentioned until now - but this is a dubious dating, read @Avery's remarks later on), that goes:

We have fallen upon evil times
and the world has waxed very old and wicked.
Politics are very corrupt.
Children are no longer respectful to their parents.

I'll let more knowledgeable contributors than myself discuss its authenticity. @Avery challenges this attribution and offers a possible explanation in his own answer. Most probably, the quote comes from king Naram Sin of Akkad, in the 23rd century BCE.

Similar complains

~2000 BCE, Egypt

• Amenemhat I was a pharaoh from the twelvth dynasty, during the 20th century BCE. He left instructions to his son Senusret, in which he notably describes his own assassination (quite a litterary feat, btw). The description contains rhetoric questions that seem to compare a deceptive present with a better past taken as reference:

Had women ever raised troops?
Had rebels ever been nurtured within the home?
Had water ever been opened up, while the canals were being dug,
And with locals at their tasks?
No disaster had come up behind me since my birth.
Never had the like happened - my moment was that of doer of valiant deeds.

The authorship of these Instructions of Amenemhat is obviously unclear, but some copies have reached us from the reign of Amenhotep I, so the text itself is at least 25 centuries old.

We can also debate if the (probably) contemporary Prophecy_of_Neferti, that describes Egypt divided in chaos after the collapse of the First Kingdom, and waiting to get unified again by, guess whom, Amenemhat I, counts as a nostalgic, before-was-better, complain.

~2500 BCE, Egypt

• Even older, the Maxims of Ptahhotep, from the 25th century BCE, contain praise for obedient youngs, even if they don't seem to complain that youngs would be less obedient now than before:

"How wonderful is a son who obeys his father!"
"How happy he is of whom it is said: 'A son is kind-natured when he knows how to listen.'"

600 - 300 BC

The counts of the indictment are luxury, bad manners, contempt for authority, disrespect to elders, and a love for chatter in place of exercise. …

Children began to be the tyrants, not the slaves, of their households. They no longer rose from their seats when an elder entered the room; they contradicted their parents, chattered before company, gobbled up the dainties at table, and committed various offences against Hellenic tastes, such as crossing their legs. They tyrannised over the paidagogoi and schoolmasters.

• “Schools of Hellas: an Essay on the Practice and Theory of Ancient Greek Education from 600 to 300 BC”, Kenneth John Freeman
  1907 (paraphrasing of Hellenic attitudes towards the youth in 600 - 300 BC)^*

“[Young people] are high-minded because they have not yet been humbled by life, nor have they experienced the force of circumstances.
...
They think they know everything, and are always quite sure about it.”

• Rhetoric Part 12 On Youthful Character, Aristotle
  4th Century BC

100 BC

“The beardless youth… does not foresee what is useful, squandering his money.”

• The Art of Poetry: an Epistle to the Pisos, Horace
  1st Century BC

Our sires' age was worse than our grandsires'.
We, their sons, are more worthless than they;
so in our turn we shall give the world a progeny yet more corrupt.

• Book III of Odes, Horace
  circa 20 BC

1300 AD

In all things I yearn for the past. Modern fashions seem to keep on growing more and more debased. I find that even among the splendid pieces of furniture built by our master cabinetmakers, those in the old forms are the most pleasing.

And as for writing letters, surviving scraps from the past reveal how superb the phrasing used to be. The ordinary spoken language has also steadily coarsened. People used to say "raise the carriage shafts" or "trim the lamp wick," but people today say "raise it" or "trim it." When they should say, "Let the men of the palace staff stand forth!" they say, "Torches! Let's have some light!" Instead of calling the place where the lectures on the Sutra of the Golden Light are delivered before the emperor "the Hall of the Imperial Lecture," they shorten it to "the Lecture Hall," a deplorable corruption, an old gentleman complained.

• Tsurezuregusa (Essays in Idleness), Yoshida Kenkō
  1330 - 1332 AD

Later than these, there are many examples of this thought being echoed from the 17th century onwards.

Note:

Sources:

^{• http://www.bbc.com/capital/story/20171003-proof-that-people-have-always-complained-about-young-adults
 • http://mentalfloss.com/article/52209/15-historical-complaints-about-young-people-ruining-everything
 • https://quoteinvestigator.com/2010/05/01/misbehaving-children-in-ancient-times/}