Genesis 5:4

Gen 5:4 And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:

Ver. 4. He begat sons and daughters.] Philo and Epiphanius give him twelve sons; Beda thirty sons, and as many daughters.

John Trapp, A Commentary or Exposition upon All the Books of the Old and New Testament (1647–1656) — public domain. Text from a community e-Sword module.

4. And the days of Adam after he had begotten Seth. In the number of years here recorded we must especially consider the long period which the patriarchs lived together. For through six successive ages, when the family of Seth had grown into a great people, the voice of Adam might daily resound, in order to renew the memory of the creation, the fall, and the punishment of man; to testify of the hope of salvation which remained after chastisement, and to recite the judgments of God, by which all might be instructed. After his death his sons might indeed deliver, as from hand to hand, what they had learned, to their descendants; but far more efficacious would be the instruction from the mouth of him, who had been himself the eyewitness of all these things. Yet so wonderful, and even monstrous, was the general obstinacy, that not even the sounder part of the human race could be retained in the obedience and the fear of God.

John Calvin's Commentaries (1540-1563) — public domain. Text from the Calvin Translation Society English edition (1840s-50s) via the Christian Classics Ethereal Library (ccel.org). Calvin's coverage is partial (roughly two-thirds of the Bible); uncommented books and verses are honest gaps, never fabricated. The 19th-century editor's footnotes and remarks are omitted so the notes are faithfully Calvin's own voice.

And the days of Adam, after he had begotten Seth, were eight hundred years,.... The Septuagint version is seven hundred; for having added one hundred years more the should be, to the years of his life before the birth of Seth, here they are taken away to make the number of his years complete:

and he begat sons and daughters; not only after the birth of Seth, but before, though we have no account of any, unless of Cain's wife; but what their number was is not certain, either before or after; some say he had thirty children, besides Cain, Abel, and Seth; and others a hundred (c). Josephus says the number of children, according to the old tradition, was thirty three sons and twenty three daughters. (d)(These families had at least five children, for one son is named as well as other sons and daughters. Therefore there must be at least three sons and two daughters in each family. For a family to have at least three sons and two daughters, according to the laws of chance, a family must on the average have nine children for this to be a near certainty. Hence the families listed in this chapter must have been large by today's standards. Given their long life, this is not at all unusual. However even today, the Old Order Mennonites of Waterloo County in Ontario and Lancaster County in Pennsylvannia, have many families this large. Ed.)

(c) Shalshalet Hakabala, fol. 74. 2. (d) Joseph. Antiqu. l. 1. c. 1. footnote on point 3.

John Gill's Exposition of the Whole Bible (1746–1763) — public domain (CC PDM 1.0). Text via the HelloAO Bible API.

Whose names and numbers are here passed over in silence, as not belonging to the genealogy of Christ, nor to the following history.

Matthew Poole's English Annotations on the Holy Bible (1683–1685) — public domain. Text from a community e-Sword/MySword module.

AND THE DAYS OF ADAM AFTER HE BEGOT SETH WERE EIGHT HUNDRED YEARS. Because of the long lives of these first men, Scripture states their ages before they begot children and also afterwards, and then sums them all up in the end until the generations which followed the flood.The reason for their longevity is that the first man, the handiwork of the Holy One, blessed be He, was made in absolute perfection as regards beauty, strength, and height. Even after it was decreed upon him that he be mortal, it was in his nature to live a long time. But when the flood came upon the earth, the atmosphere became tainted, and as a result their days kept on decreasing. Until the flood, their days were about the length of Adam’s; some even lived longer than Adam. And Shem [Noah’s son], who was born before the flood, lived six hundred years; he benefitted from his innate strength, but the tainted air after the flood caused him harm, [hence he died at a younger age than that attained by the preceding generations]. The days of his sons who were born after the flood were still more shortened until they came down to four hundred years., Verses 10-17. You can see that this degree of longevity remained with them until the generation of the Dispersion, when the change of climates caused by the Dispersion affected them, and their days were again shortened. Thus you find that the life of Peleg, in whose days the earth was divided, 10:25. came down to half their days, i.e., two hundred years., 11:19.It would appear that in the generations of Abraham, Isaac, and Jacob, people lived seventy and eighty years, just as Moses, our teacher, mentioned in his prayer.. But as for the righteous ones in their generations, The fear of the Eternal prolongeth days for them. For Pharaoh wondered about Jacob’s old age, and Jacob in turn spoke to him about the long days of his fathers, even as he said, And they have not attained unto the days of the years of the life of my fathers in the days of their sojournings. Now what the Rabbi has written in the Moreh Nebuchim does not seem right to me, namely, that the longevity was only in those individuals mentioned, while the rest of the people in those generations lived lives of ordinary natural length. He further said that this exception was due to the mode of living and food of such people or by way of a miracle. But these are words without substance. Why should this miracle have happened to them since they were neither prophets nor righteous, nor worthy that a miracle be done for them, especially for generation after generation. And how could a proper mode of living and proper food prolong their years to the extent that they are so many times greater than that of the entire generation? It is possible that there were also others who observed such a mode of living, in which case all or most of them should have attained similar longevity. And how did it happen that enough of the wisdom concerning this good mode of living did not come down to just one of all the sons of Noah after the flood [to enable him to match the longevity of his ancestors], for there was among them a little wisdom of their ancestors even though it steadily decreased from generation to generation?

Ramban (Nachmanides) on the Tanakh — English translation: “Commentary on the Torah by Ramban (Nachmanides). Translated and annotated by Charles B. Chavel. New York, Shilo Pub. House, 1971-1976” (CC BY 4.0). Text via Sefaria.

ויהיו ימי אדם, it appears that all the people mentioned prior to Noach, whose lifespan is given by the Torah, were included in the ones described as being born בדמותו כצלמו, as reflecting the qualities expected of a human being in the true sense of the word, whereas their brothers and sisters did not. This is why they were wiped out during the deluge. It is possible that the long lives of the people mentioned in this list was a standard set by G’d at the time, and shared by their contemporaries, their siblings. On the other hand, it is possible that only the select list mentioned by the Torah enjoyed such an exceedingly long life on earth. Perhaps the reason G’d granted those generations such long lives may have been to enable them to study phenomena over a long period of time and to record the results of their observations for posterity. A lifespan of 70 years as we know it today is simply not long enough to accumulate this type of knowledge. Once these basic data had become known and recorded, later generations could study them out of books, making it unnecessary for them to live for so many years. The outstanding scholar Rabbi Moses Maimonides wrote in his Moreh Nevuchim 2,47 that only the men mentioned by name in our chapter lived to such an old age. He goes on to write that exceptional sizes, years, and numbers mentioned in the Bible are all just that i.e. exceptional. They do not purport to describe the norms in the time or environment during which such exceptions occurred. There were varying sets of circumstances which accounted for these exceptions.

Radak on the Tanakh — English translations: “Sefaria Community Translation” (CC0 1.0 (public-domain dedication)); “Eliyahu Munk, HaChut Hameshulash” (CC BY 4.0); “Rabbi Mike Feuer, Jerusalem Anthology” (CC BY 4.0); and “R. David Kimhi on the first book of Psalms, Translated by R.G. Finch, London, 1919” (public domain). Text via Sefaria.