On my path to reading the Bible, one of the first things that stumped me right out of the gate was the tremendous life spans of the Patriarchs listed in the Bible. I tend to view things with a scientific lens, so I was immediately starting to live up to my name as a “doubting Thomas.” I started doing some research, using good ol’ Google, and found some interesting websites that had some fascinating theories.
The first was: https://answersingenesis.org/bible-timeline/genealogy/did-adam-and-noah-really-live-over-900-years/
I highly recommend you read it. It discusses the theological reasons for the human life span being reduced. Stemming from Adam eating the forbidden fruit so that he would eventually die along with his decedents (Genesis 2:16-17) to scientific reasons as to how it could have happened. They discuss how environmental changes could have occurred after the Flood that would have possibly shortened human life spans, and then also genetic bottle necks that would have resulted from a drastically reduced population. They also hint about other literature from that time-period that discusses significantly longer life spans in other cultures.
The next was: https://rcspirituality.org/ask-a-priest-did-noah-really-live-900-years/
This is a response to a similar question by a Catholic priest. While I wouldn’t consider myself a Catholic (I attend a Baptist Church, but currently would not strongly align myself with any particular religion at this point in my religious journey-.) He makes some good points. The line that sums up his response is “Recent Church teaching has leaned toward interpreting Genesis 1-11 as stories that, while they have some basis in history, are meant primarily to teach theological and spiritual truths.” I encourage you to read this article, as I found it to be quite insightful.
This site http://www.ancient-origins.net/human-origins/did-ancient-people-really-have-lifespans-longer-200-years-002093 is a non-Christian source that discusses other cultures that also record incredible declining life spans.
Another very interesting article: http://creation.com/living-for-900-years This site discusses mostly scientific reasoning for the long lives of Biblical times. They go through environmental reasoning, along with genetic theories that are well articulated.
This site http://www.reasons.org/articles/why-aren-t-there-any-900-year-old-human-fossils asks the question “Why aren’t there any 900 year old human fossils?” Explaining their theories as to why.