I do not know where I read this idea - but it is not my own.
For some of God's decrees, there is a four-part implementation:
- Decree given
- Example made
- Waiting period (for mercy)
- Full implementation
The first example is the prohibition against eating from that tree in Eden.
Adam and Eve were told that "on the day you eat of it, you shall surely die". They did not die on that solar day - they died near the end of that prophetic thousand-year day:
A thousand years in your sight are like a day that has just gone by,
or like a watch in the night. (Psalm 90:4)
So the decree was God giving the law against eating the fruit, the example was the death of Abel at the hand of his brother Cain, proving that death had entered the world, the waiting period for mercy was about 930 years, nearly a whole prophetic 1000-year day.
Now we turn to the days of Noah and God's decree about shortening life further.
The decree was given in Noah's day, the example was the flood, which came 120 years later, showing that God was not messing around. Then we enter the 1,000 year waiting period. In mercy, God permitted many righteous people (like Job, Abraham, and other patriarchs) to live more than 120 years, but the maximum life expectancy kept dropping, showing a phased implementation. Then comes the kicker: if you follow the chronologies, the 1,000 year period of grace ended about two decades before Moses died. Moses was full of vigor and in perfect health when he died:
7 Moses was a hundred and twenty years old when he died, yet his eyes
were not weak nor his strength gone. (Deuteronomy 34:7)
Moses is the last person in the Bible to live to 120 years. No one subsequent exceeds his age. Claims in human history since then of people living longer cannot be substantiated. In recent times, proper documentation is required by the groups that track records (like the Guinness Book of World Records). Since that started, claims of people living longer have been debunked.
Thus Moses - who received the Law (like the 10 Commandments) from God, also received the law of human longevity in his own person, being shown as an example of God's law entering its period of universal effectiveness.