Tons of baseless speculation to follow:
It's interesting to think that older people in ancient times on average were "healthier" than older people now. Infectious disease and death by violence were much more prevalent then. But if you could avoid deadly infectious disease, violent death, cancer and things like appendicitis, then you were probably in top shelf condition. Salt was expensive, people weren't obese, tobacco wasn't a thing yet, people didn't add sugar to everything, air quality was great outside the cities. I read somewhere that tobacco and obesity alone account for 80% of cancer and probably pretty close to that for heart disease and stroke.
As far as people living 900 years, I have a completely baseless idea that it refers to nations. After all, you could legitimately say that Israel was born, gave birth to Judah after a few hundred years and died after about 400 years. You could say Judah was born and died 500 years later. Like I said, really nothing concrete to suggest this, but all the Hebrew nations were named for their patriarchs. So Israel refers both to Jacob and the nation. Judah refers to the same. Problem with this reasoning is that you run into Noah, and he was 500 years old when the flood happened.
I've also seen speculation that they were counting months, which would have made Methusaleh about 80 at death and Noah about 50 at the time of the flood. But then you get people becoming fathers around age 5 or 6, which doesn't make sense either.
Or maybe people really did live that long, but when God flooded the Earth he permanently changed something like the level of oxygen in the air or the ozone layer or introduced a virus or mutation into Noah's kids and that shortened human lifespan.
In regards to Moses being 120 and vigorous, he was the only person to see God face to face. He actually glowed afterward, so maybe that did something for his vitality.
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