There are lots of theories to explain why the men listed in Genesis lived so long, from the “clean environment = longevity” theory, to the “ages = clan/dynasties” theory, to “somebody added a zero/scribal error” theory. My favorite is the theory is that the ages were “sacred numbers” or symbolic somehow. The ancient Mesopotamian numbering system was based around the number 60 (most likely due to how they calculated astrological situations), which meant their highest god was assigned the ideal number, 60. Of secondary importance was the number 10, which was the number assigned to mankind (5 fingers on each hand meant a whole man = “10”). So, taking these two basic concepts, when we are told Noah was 600 when the flood came, and that he was “righteous” (or right with God) we can see that he was “10 X 60” or, 600. There’s an interesting article on this point of view here.