Problem: Did people really live hundreds of years according to Genesis? Yes, they did. But, why?
Solution: Adam lived 930 years: Gen. 5:5, “And all the days that Adam lived were nine hundred and thirty years: and he died.”
Seth lived 912 years: Gen. 5:8, “And all the days of Seth were nine hundred and twelve years: and he died.”
Methuselah lived 969 years: Gen. 5:27, “And all the days of Methuselah were nine hundred sixty and nine years: and he died.”
After the fall, the genetic line of Adam and his descendants was very pure, so their health would have been incredibly long. Some theologians think that there was a canopy of water that engulfed the entire earth and that it was released at the time of the flood. “In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month, on the same day all the fountains of the great deep burst open, and the floodgates of the sky were opened,” (Gen. 7:11). The “floodgates of the sky” are sometimes alluded to as great amounts of water suspended in the sky. Also, no rain is recorded in the Bible until after the flood which seems to support this idea. This canopy, if it is true, might have provided some sort of protection from the sun’s harmful rays. We can’t know for sure, and it is only a theory. Nevertheless, after the flood, the lifespan of people on earth was drastically reduced. “Then the LORD said, ‘My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years,’” (Gen. 6:3). Whether or not this reduced canopy had any effect on the human lifespan may never be known.
But, we can conclude that they lived a long time in Genesis because they were so close to the original creation, that their biological purity was far greater than ours is today.