Many theologians and such throughout the years have interpreted Gen 6:3 to mean that humans only have 120 years to live, however, many post-flood people lived way longer than that, Terah: 205 (Gen 11:32), Abraham: 175 years (Gen 25:7), and Isaac: 180 years (Gen 35:28). What does this mean?
Gen 6:3 (NLT) Then the Lord said, “My Spirit will not put up with humans for such a long time, for they are only mortal flesh. In the future, their normal lifespan will be no more than 120 years.”
Gen 6:3 (KJV) And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
There is another possible interpretation of the 120-year limit mentioned in Genesis 6:3. In Gen 6:3 God says his spirit will not dwell with man forever, he also is flesh, and his days will be 120 years. It is mentioned in the Dead Sea Scrolls that God said this when Noah was 480 years old. 120 years later Noah was 600 and that is when God sent the flood.
“In the year four hundred and eighty of Noah’s life, Noah reached the end of them. And God said: ‘My Spirit will not reside in man forever. Their days shall be fixed at one hundred and twenty years until the end of the waters of the flood.’” – Source: [4Q252 1.1-3, see Devorah Dimant, “The Flood as Preamble: The Perspective of the Qumran Hebrew Texts,” pp. 110-122]
Whether or not this other flesh is talking only about animals and humans or includes the Sons of God (rebels of heaven) could be debated. No matter, if Gen 6:3 was spoken by God when Noah was age 480 then that would mean that this was at least a countdown before the flood would come and wipe out all flesh that can’t survive in water for at least a year. There is a hypothesis about the phrase “he is also flesh” suggesting that this is in reference to the Sons of God and not humans because they are the new kind of flesh that “also” existed alongside humans and they would be killed along with humans and that is why they are mentioned in the next verse.
Gen 6:4 (NLT) In those days, and for some time after, giant Nephilites lived on the earth, for whenever the sons of God had intercourse with women, they gave birth to children who became the heroes and famous warriors of ancient times.
There were new giants emerged after the flood in the land of Canaan. It is unknown if this is because the flood was local and they along with other people survived or if they were born with the same kind of forbidden intercourse post-flood. In the case of a global flood, new heavenly beings could have mated with humans, or maybe their genes are carried by women and were carried over by Noah’s daughters-in-law.
Either way, the point is that the Dead Sea Scroll offers a different perspective from first-century Jews on the understanding of Genesis 6:3 that gets around the fact that patriarchs after Noah, including Job, live way longer than 120 years. Even patriarchs just before Moses lived past 120 years. According to Exodus 6:16-20, Moses’ father Amram lived until 137, and Kohath his grandfather until 133, while Levi his great-grandfather lived until 137. Moses is the first person recorded whose death age is exactly 120. Moses’ brother Aaron died the same year as Moses at 123 years old.
The mainstream theory is built on an assumption that God meant 120 years to be a goal achieved gradually thing over time unit the time of Moses since the ages of the patriarchs are decreasing from Noah to Moses. Moses is the last biblical figure mentioned (excluding Job) that lives that long. The Dead Sea Scrolls, which go back to around the 4th century BC, weren’t discovered until 1948, so the KJV authors and other European church fathers through the ages may not have had access to this view or may have rejected it. Long story short there are two interpretations, so pick whichever one you believe is correct until God reveals more info.
4Q252 1.1-3, see Devorah Dimant, “The Flood as Preamble: The Perspective of the Qumran Hebrew Texts,” pp. 110-122