There is a concept often talked about in orthodox circles called yeridas hadoros. What is the earliest source for this idea? Does anyone argue?
The question is rather vague, including any sort of decline in generations, but here are a few sources that indicate this, or examples of the contrary.
This idea is indicated in several places in Hazal. For example, in Yevamot 39b, Rav Nahman bar Yitshak incredulously asks: "have the generation improved?". His incredulousness at the possibility strongly indicates that he assumed a downwards progression, although he could have been convinced it was merely stagnant.
A clearer formulation is found in Shabbat 112b:
אמר רבי זירא אמר רבא בר זימונא: אם ראשונים בני מלאכים - אנו בני אנשים, ואם ראשונים בני אנשים - אנו כחמורים
Rabbi Zeira said in the name of Rava bar Zimunna: if the early ones were the sons of angels, we are sons of men, and if the first ones were sons of men, we are like donkeys.
There are sources that indicate otherwise, however, that there is improvement at least among some portions of the population, and in some respects. For example, the Jews greatly reduced their idolatrous activities, cf. Sanhedrin (64a) which depicts the loss of the yearning towards idolatry.
More broadly, some thinkers see the Torah as perfecting society on an ongoing basis.
In this spirit, Rav Kook (cited here) writes that the laws regulating the capture of captive women eshet yefat toar, and the laws regarding slavery were concessions to the times, but not ideals, and later generations should avoid them.