In Gen 6:3 God declares man will no longer live longer than 120 years. How is this number not considered a generation since, in theory, it is possible for man to live that long. I think, based entirely on just personal observation not science, that man’s life span has been increasing related to quality of health care, diet and other factors which make us healthier than at any other time since the flood. So why is this number not considered when looking at the time frame from the establishment of Israel as a nation or the retaking of Jerusalem?
First of all a generation is the span of time between a man’s birth and the birth of his first child. Genesis 6:3 is talking about lifespans.
That said, many scholars believe that Genesis 6:3 is actually a 120 year warning of the coming flood. The reasons for this are:
1) the long life spans common before the flood continued for several hundred years afterward. The first post flood patriarchs lived for 400 years or more. Abraham was born nearly 300 years after the flood and lived for 175 years. Joseph was 110 when he died and Moses lived for 120 years but lifespans kept dropping after them.
2) In Psalm 90:10 an average man’s life span is clearly given as 70-80 years, contradicting the 120 year interpretation of Gen 6:3, and
3) the fact that with all the advantages of modern medicine hardly anyone lives to 120. And even though some countries currently experience long lifespans, the world wide average is still slightly below 70. Once lifespans dropped below the 120 year mark no ethnic group in the past or present has ever maintained an average lifespan of such length. Today the Japanese live the longest, an average of 79 years for men and 85 for women.