Genesis 5:4

And the days of Adam after he begat Sheth were eight hundred years, and he begat sons and daughters.

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?

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.