"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)
No need to even leave the verse to answer this question, thank the Lord.
A common misconception is that here God decided to limit man's lifespan to 120 years. While it's true that lifespans decreased exponentially after the flood (see Gen. 11), a few people were living to 120 many centuries after the flood (e.g. Moses--Deut. 34:7; Jehoiada--2 Chron. 24:15). Two key words in the first part of the verse (v. 3a) unravel the second part (v. 3b): "not always." God is putting a limit on the time he will continue to strive with men (spiritually) to lead them to repentance (Rom. 2:4; 2 Pt. 3:9; Rev. 2:21). An example of this is Jonah's preaching to the Ninevites, in which God gives them 40 days to repent (Jon. 3:4). Since God publicly announces the "space to repent" for Nineveh, he may have done the same before the flood through Noah, "a preacher of righteousness" (2 Pt. 2:5) and his final voice of warning before the flood, since Enoch, also a preacher (Jude 14-15), had disappeared 134 years before Noah's birth.
A final thought in support of v. 3a expounding v. 3b is Gen. 6:7, where God determines to destroy mankind, pointing to the 120 years as a probation period, after which God decides to act. Man's wickedness (6:5) was already grieving the Lord (6:6) at the time the probation period is announced, and since mankind does not repent at Noah's preaching (unlike the Ninevites!), judgment comes after 120 years.
Yesterday I saw a rainbow while driving, and it reminded me that "in wrath" God "remember[ed] mercy" (Habakkuk 3:2) during the flood (Gen. 6:8, 8:1), NOT about a group of sinners (1 Tim. 1:9-10) "whose glory is in their shame" (Phil. 3:19). They are the losers, dear friends, not champions, if they refuse God's glorious gospel (1 Tim. 1:11) in the time THEY have left.
