“…nevertheless his days shall be one hundred and twenty years.”
This passage has commonly been understood one of two ways: 1) As an indication of the amount of years left between God’s decision to do something about man’s wickedness and the start of the flood; or 2) A limitation on the lifespan for humankind, so our age would not exceed 120 years. Let’s explore both interpretations.
120 Years Before the Flood
We do know that 100 years passes between the time Noah has his children (Gen. 5:32) and the time the flood begins on the earth (Gen. 7:6). Additionally it seems plausible that God’s proclamation in this verse could have taken place 20 years prior to the birth of Noah’s children, as Genesis 5 is essentially a listing of descendants and does not necessarily come before Genesis 6 chronologically. If so, this could account for the 120 years. Noah would have needed some time to build the ark of course; though at some point his three sons could assist. With this interpretation, the 120 year waiting period is also considered to be due to God’s grace and mercy, and as such would probably be understood as a time to repent. However, we read nothing in the text of repentance of others, nor even of Noah or his family calling others to repent with the flood coming. This may seem unkind or odd, however it brings to mind a verse from Revelation 22:11, which states:
“Let the one who does wrong, still do wrong; and the one who is filthy, still be filthy; and let the one who is righteous, still practice righteousness; and the one who is holy, still keep himself holy.”
The above words are explained to St. John after his vision of the apocalypse, and infer that at that point it is too late to make any behavioral changes as Christ would be returning soon (from the standpoint of the vision.) Perhaps this was the case with Noah; God may have informed him not to call others to repentance as judgment was nigh. Either that, or any calls to repentance are simply not recorded. Even so, ultimately no one else was saved in the ark aside from Noah and his family.
Lifespan Reduced to 120 Years
Alter’s commentary points out this reduction in man’s lifespan isn’t the first, but the second; the first having been pronounced at the expulsion from the garden, at which point man was prevented from living forever by losing access to the tree of life. Now after 10 generations of patience after the first sin of disobedience, during which time the whole earth became corrupt and filled with violence, (Gen. 6:11) God hands down another reduction in our years. Why? Because look what happens when we live so long – we finely craft our wickedness evermore. As God said in Gen. 6:5, every intent of the thoughts of our hearts is evil continually. Thus it would stand to reason that a shorter lifetime means a lesser degree of wickedness we can attain to (or you might say, the less trouble we can get ourselves into.)
It is worth noting that at the end of the Torah, Moses lives to the age of 120. Moses’ age, coupled with the fact that in modern times we still do not live past 120, is a strong argument for this interpretation. Ultimately either explanation of the 120 years is plausible, however given the context I would lean toward the reduction of lifespan.