Cameron B. answered 03/22/21
MA.'s in Church History & Theology; Minor in Biblical Studies
A key and important detail to point out in order to answer this question is that only certain people/individuals in the Old Testament, in particular the Pentateuch, were said to live an unusual amount of time. The reason for this is very clear, especially when one considers the Genealogies listed in Genesis 4 & 5.
In Genesis 4, Cain's descendants are the focus. No dates or lifespans are given and this is intentional of the author. Instead, details concerning Cain's descendants highlight actions such as builders of cities, weapons crafters, and polygamy, all of which would have been associated with kings in the Ancient Near East.
In contrast, Seth's descendants in Genesis 5, are highlighted by their longevity and their devotion to God. They are not perfect, as is also clear by later followers of YHWH, such as Abraham and Moses. Longevity and the blessings they receive is not a consequence of their perfection but rather of their devotion to God and his laws. It is clear in this case that the longevity of Seth's line is meant to be interpreted as divine blessing, not a universal standard.
With that said, it is possible that the unusual lifespans should not necessarily be taken too literally. It is not uncommon for generations to be skipped or dates to be tweaked in order to express a theological purposes (cf. Matthew's Genealogy of Jesus not only skips many generations, but scholars have pointed out that before David, there is a perfect seven [7] generations and after David is another perfect seven [7]. This would seem to imply that Matthew intentionally was choosing and designating a specific number of generations as well as which generations he was going to highlight). Also, dates in the ancient world were difficult to ascertain with certainty. It was not uncommon for people to choose a starting date not associated with birth. It is quite possible that the author of Genesis used the dates for more theological rather than historical purposes. For readers of the Modern age this may seem odd, but for most of the Ancient Near East, this was commonplace in genealogies and historical narratives. But, then again, maybe people did really live several hundreds of years. Some things we may never know with 100% certainty.
Hope that this helps!
Cameron Brock