Some times you see all kinds of crazy explanations to the lifetimes of the Biblical patriarchs from 1. Moses. Some suggest that they are mythical, others that maybe the atmosphere was different and people lived longer. The correct explanation is simpler: they are intentionally selected numbers that contain the Jewish Messianic calculation of that time.
The lifetimes of the patriarchs in the Bible are very important and they are not to be understood in a naive way. They contain the old way how to calculate the coming of the Messiah, that is, the end of times.
Firstly, look up from Wikipedia the Sumerian king lists and compare them to the genealogies of the patriarchs in the Bible. Take the genealogy of Cain from Septuaginta (you find it in English from the web) from the place 1. Moses 4:17–24 and compare it to the kings of the 1. dynasty of Uruk. You will see very clear similarities. Both lists have characterizations, such as smith, shepherd, builder of a city, inventor of poetry or music. Both start from a son of a god (Adam, created by God and Mesh-ki-ang-gaster, son of a god). You will notice that Enoch corresponds to Dumuzid.
Let us take another example, compare 1. Moses 5 with the kings of Sumer. Here Lemek, Noah’s father, corresponds to Ubara-Tutu, the father of the Sumerian Noah, Atra-hasan. This is natural, since the story of Noah’s deluge is a later version of the story of Atra-hasan’s flood in Shurupakk. In this list Adam corresponds to Adapa. Adapa is a son of god. The Sumerian king lists contain only two sons of a god (a different god), Mesh-ki-ang-gaster and Adapa. In the Bible, which is monotheistic, both become the first man created by God. In this patriarch list the order of Sumerian kings is a bit changed, but with some matching you find that Dumuzid here corresponds to Enos. This is not the same Dumuzid as before (that was the fisher, this is the shepherd), and this is Enos, not Enoch. Dumuzid the shepherd become a god, Tammuz. Enos was the first great prophet and for Jewish Christians of the 1. Century he was one of the incarnations of Jesus. At the time of Enos people started calling for God, says the Bible. Enoch, on the other hand, did not die. He was taken to Heaven. Later he was identified by the angel Metatron, the voice of God.
There is also a genealogy starting from Noah. It corresponds to the king list of Akkad, starting from Sargon of Akkad.
To the question why Adam has a 900-year lifespan, in the king lists the ruling times are given in sars and ners. Later sar was understood to mean 3600 years and ner 600 years, but originally they most probably meant something else. The ruling times in the king lists are still much longer than the life times in the Bible, but it seems that the writer of the genealogies of patriarchs knew the king lists and took the times in sars and ners and understood that Sumerians had originally used a moon calendar and before that a day calendar. He converted the times and obtained lengths that are rather similar to the ones in the Bible (in Septuaginta). I did this calculation, the details are in the text in Finnish. The times so obtained are not exactly the ones in the Bible because the order of the kings is changed in some places in the lists of patriarchs. The reason for this is that the writer of the lists wanted to hide the 6000 year plan to these numbers.
The 6000-year plan is that the world exists for 6000 years, first there are 2000 years without law, then 2000 years of law and then 2000 years of the time of Messiah, the whole cycle is 7000 years, after 6000 years follows 1000 years when only God is strong. This is the basic Jewish theory of eras (you find it in Talmud Sanhedrin 97a, Abodah Zarah 9a and in Tanna debe Eliyyahu)
In this theory, every 2000 years is divided into two 1000 years parts and there is a prophet at every important time. The years do not start from Adam. They start from Set (third from Adam), then to Enos (in whose time people called for God), then to Enoch (who did not die), then to Noah (who was given the law). Originally the time of Law was supposed to start from Noah and the time of Messiah from David, but it went as went and the times were moved forward on several occasions.
Why then Adam has the lifespan that he has? It is (and use Septuaginta) in order to have the times match. From Set to Enos must be 1000 years and so on. At some point calculation was started from Adam and that fixed his lifespan.
It was not allowed to change the lifetimes of the patriarchs, nor the times when they got the son. It is because the whole Jewish calculation of the arrival of the Messiah was based on these times. But around 800 AD the Masorites changed the times. The Christian Bible (apart from Orthodox) is Masoretic, so it is falsified. If was falsified because Jesus fitted too well the times of Septuaginta.
The patriarch lists are in the Torah (Genesis) meaning that they were written most probably in the Babylonian captivity (when Jewish priests had access to Babylonian archives containing Sumerian king lists). From the 6000-year plan all but the Messiah had already been fulfilled. This is why they were just given as events passed: Set-Enos-Enoch-Noah-Abraham-David/Temple-Messiah. There is 1000 years between each important person. We see that the Messiah should have appeared 1000 years after David, or after the temple. Josephus gives the building of the temple as 968 BC, count 1000 years from that gives 33 AD, but the Messiah could have been a bit earlier or later. One could always select the important events a bit differently. Probably this is the reason for the tradition that Jesus was crucified in 33 AD, while modern researchers opt for 30 AD.
To summarize, the lifetime of Adam is not a myth, it is secret knowledge intentionally hidden there. The time does not intend to claim that humans once lived so long. Indeed the Bible says that the human life was limited in the deluge to 120 years and soon after that gives a person after the flood that lived longer. This shows that the times are not meant to be read as life lengths. It is also not correct to say that the years used to mean months. That is the origin, in Sumerian king lists, but presently the times are selected to give the 6000-year plan. The times are prophetic. They are a way to calculate the coming of the Messiah, but unfortunately the method is outdated. All times went already and Jesus was the last who fitted the plan. This is why the rabbis say that those times have passed long ago and the Messiah did not come.