In regards to a generation of years, God stated that no one would live longer than 120 years after the great flood. As far as I am aware some individuals have come to live very close to 120 years but not to 120. So does this mean that the average life span could be longer than 70-80?
The Bible does not say man’s life span would be limited to 120 years, and in fact, several post-flood generations lived longer than that. Genesis 11:10-32 recounts the generations from Shem to Abraham, all of whom lived substantially longer than 120 years. Abraham himself lived to be 175 years old (Genesis 25:7). The 120 year period mentioned in Genesis 6:3 was the length of the warning God gave mankind that the flood was coming to destroy them.
Had 120 years been the God-ordained lifespan of mankind, then Moses, who wrote Psalm 90, could not have said, “The length of our days is seventy years – or eighty if we have the strength” (Psalm 90:10), and Isaiah could not have said that seventy years is a king’s lifespan (Isaiah 23:15). These are the only places I’m aware of in the Bible where an average human lifespan is measured.