I had one person explain it to me this way: Death is a consequence of sin. If you have no sin, there is no death (cf. Enoch, Elijah, Jonah, Jesus, also Romans 6:23, Genesis 2:17).
Consequently, the magnitude of the sin we carry within us affects how fast we die, like a disease. Adam and Eve sinned, but the magnitude of their sin was small relative to the really creative and self-destructive ways we’ve invented for sinning in the modern era. The truly heinous ways of sinning developed gradually over time.
As this happened, man incorporated a greater degree of sin into his organism, and his lifespan became shorter and shorter, until we finally get to the modern era, where it is exceedingly rare for a person to live past 100, and even those who do live to that age, are typically very decrepit or sickly, and are only kept alive by some artificial chemical soup that has to be constantly pumped into their bloodstream, and if they forget to take their pills for a day they will die.
In that sense, I view it as entirely plausible that the earliest individuals cited in the Bible lived for very long times.
