You have said, “According to Psalm 90:10 an average Biblical life span is 70 years, eighty if we are strong.” It stand in Gods Word and is therefore authentic. But God Himself said something different in Gen 6:3 “And Jehovah said, My spirit shall not always strive with man, in his erring; he is flesh. Yet his days shall be a hundred and twenty years.” As far as I know, no where in the Bible has God reversed these words. How do you reconcile this?
Since both statements are in the Bible they must both be correct, so one of them has to be referring to something else. A look at Bible history tells us that pre-flood lifespans were much longer than 120 years, and even after the flood, they remained so. Genesis 11:10-32 shows lifespans well above the 120 year mark, generations after the pronouncement of Genesis 6:3. For exmpe Abraham lived for 175 years (Genesis 25:7). His grandson Jacob lived to be 147 years old (Genesis 47:28).
Life spans decreased steadily after that until they finally leveled off in the 70-80 year range among the Jews where they’ve been for several thousand years. Until about a hundred years ago Gentiles have typically died at a much younger age. Today, even in countries where health is good, a man reaching 100 years of age is considered a rarity.
So then, what was God talking about in Genesis 6:3? Many scholars, myself included, feel that in Genesis 6:3 God was proclaiming a grace period of 120 years between His announcement of a coming judgment and its actual arrival. He was speaking of the Great Flood. This view seems to be confirmed by the context of Genesis 6 and eliminates the conflict between Genesis 6:3 and Psalm 90:10.