Consider another translation: “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.” (Genesis 6:3, KJV).

This verse is not specifying a maximum human lifespan, but the period of probation in which God's spirit would strive or plead with mankind before their judgment by the Flood. This is the period during which “the longsuffering of God waited in the days of Noah, while the ark was a preparing” (1 Peter 3:20).

Genesis records numerous descendents of Noah who lived longer than 120 years (see Gen. 11:10-25). Lifespans did decline rapidly after the Flood, however, and it was recorded in Psalms that “The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away.” (Psalms 90:10).

Even today with our advanced medical knowledge and technology, life expectancy is still typically between 75-85 years.