In the old testament we find people with huge life expectancy, but as time passed life expectancy came down, why?

The sacred author is trying to demonstrate a theological point: as the sins of humanity got worse and worse over the years, the effects of sin increased as well. And the worst consequence of sin is death. As sin got worse, death entered the world more and more. People’s lives getting shorter and shorter is meant to be a sign of the effects of sin in the world. That’s also why especially righteous people like Noah, Moses, or Joshua had long lives by the standards of their day.

Some people also point out that it’s unrealistic to believe that someone could have actually lived for 900 years. That might be true, but the most important part is not whether or not those people actually lived that long. What’s most important is the theological message the author is conveying: the effects of sin on the human race. Plus, we really have no way to know for certain how long those people in the Old Testament actually lived.

It’s good to keep in mind, however, that many of those early chapters of Genesis were written in the style of folktales, songs, and legends–in other words, in the style of myth. That means the manner in which they convey truth is not the same as a history textbook. Today, we use the word “myth” to refer to something false–but in its original sense, myth refers to a kind of story which often conveys its truths in symbols and archetypes.

So when we say the early chapters of Genesis are told in a mythic style, we’re not saying that they’re fake; rather, what we mean is that the authors usually weren’t concerned with historical accuracy. Their concerns were about showing how in the stories that had been handed down for many generations, you can see God’s hand at work.

In simpler terms, the stories are concerned with theological truth first, and historical truth is secondary. They aren’t just stories made up out of nowhere, but they might not be historically accurate.

We should be cautious about reading the entire Bible literally. We don’t want to misunderstand the intentions of the authors. That’s also why the Church helps us to interpret the Bible, based on the wisdom of Sacred Tradition and the guidance of the Holy Spirit.