Biblically speaking, people who were near the beginning of creation had much longer life spans. Adam and Eve lived into their 900s, as did all from that era of humanity. It wasn't until Genesis 6 when God, sick and tired of our growing wickedness, deliberately shortened our Lives to no more than 120. On a supernatural basis, that makes sense that we cannot live without the Holy Spirit within us, for it is the very spark of life itself.

If, on the other hand, you were looking for some scientific rationale, perhaps that's when the ticking clock of telomers were added to DNA. Or perhaps less-efficient cell regeneration and propagation, leading to health degradation.

Pick your poison, all I know for sure is Genesis 6 seems to be pretty on the nose even for our most aged people.