If we study the lifespans of the Biblical figures from the Flood and on, they declined from 950 (Noah) to 600 (Shem) to the 400’s, and then to the 200’s.
They explain that 120 is a rough limit, not a precise one.
Take heed then to your spirit, and let no one deal treacherously against the wife of your youth.
New American Standard Bible Copyright ©1960, 1962, 1963, 1968, 1971, 1972, 1973, 1975, 1977, 1995 by The Lockman Foundation, La Habra, Calif.
The average person, knowing he had so many centuries to go, did not fear death and his ultimate encounter with God.
He had many years to live and enjoy himself first, with virtually no sense of his mortality.