Many explanations are possible - miracle, mythology, exaggeration, the lifespans were severely shortened since them, etc.. It is useful to compare to the antediluvian Sumerian King Lists, which also had long reigns for kings (around 30,000 - if we take the base system to be base 60), but the conclusion of such a comparison is not obvious.
However, some people see these large numbers and decide that the first few perakim of Bereishit must be nonfactual. (This despite the fact that they also would adopt the Documentary Hypothesis, which would divide these extreme years from accounts of other events.)
I don't know if anyone has suggested this yet, but here is one of several ideas I've had on the matter.
How do we know the meaning of words? We see the words in common usage. Yet sometimes meanings of words change over time, and if we ascribe the new meaning to the old word, we end up with nonsense. Consider the intro to the Flinstones where they "have a gay ol' time." Or consider the archaic usage of "awful," to mean "inspiring awe or admiration or wonder." The gemara gives a list of words that have changed meanings over time, as I discuss in this blogpost on the permissibility of circuses.
How do we know that shana means year? It usually means lunar or solar year, which consists of 12 months. But do we know that it always meant that? Shabbat occurs every 7 days in Hebrew, but in Akkadian the possibly unrelated word shappatu occurs once a month, on the 14th or 15th day.
Another thing to note is the ratio between ages of having a first child and total lifespan. While the people in the beginning of Bereishit live much longer lives, they have their first child relatively late - around 100 to 13o years old. Meanwhile, people who live later on have shorter lifespans but have their first child much earlier.
Perhaps this is a deliberate artifice, because otherwise they would not live long after having their children. However, why assume so off the bat?
Specifically, let us assume that shana comes from the word repeat (shny, and not from yshn), and is thus related to the word shnayim, two. Further, let us assume that in the time in which the initial genealogical list were recorded, chodesh is a month, and shana is a two-month period. We could say this even without the etymology. If so, we would need to divide each "year" by 6 to get the actual year. (This is just an exercise to see if it works out. I have know way of knowing exactly how long a "year" was.)
What do we have, then?
Adam begat Shet at 130 years.
130 / 6 = 21.66
and died at 930
930 / 6 = 155
Shet begat Enosh at 105 years
105/ 6 = 17.5
and died at 912
912 / 6 = 152
Enosh begat Kenan at 90 years
90 / 6 = 15
and died at 905
905 / 6 = 150
Kenan begat Mehalalel at 70
70 / 6 = 11.66
and died at 910
910 / 6 = 151
Mehalalel begat Yared at 65
65/6 = 10.83
and died at 890
890/6 = 148.3
Yared begat Enoch at 162
162/6 = 27
and died at 962
962/6 = 160.3
Enoch begat Metushelach at 65
65/6 = 10.83
and died at 365 years, a young age (also the number of days in a year)
365/6 = 60.8
Metushelach begat Lemech at 187 years
187 / 6 = 31
and died at 969
969 / 6 = 161.5
Lemech begat Noach at 182 years
182 / 6 = 30.3
and died at 777 (he was Lubavitch)
777 / 6 = 129.5
These numbers are more in sync with other ages in Tanach. Of course, we must assume some early puberty for Kenan, Mehalalel, and Enoch, but it is more possible.
Noach appears to have had his sons at the age of 500, but do not be fooled:
|לב וַיְהִי-נֹחַ, בֶּן-חֲמֵשׁ מֵאוֹת שָׁנָה; וַיּוֹלֶד נֹחַ, אֶת-שֵׁם אֶת-חָם וְאֶת-יָפֶת.||32 And Noah was five hundred years old; and Noah begot Shem, Ham, and Japheth.|
The genealogy resumes in perek 11.
Shem begat Arpachshad at 100 years.
100 / 6 = 16.6
and lived 500 (more, presumably) years.
600 / 6 = 100
We actually do not know if this is total age or age after first begetting, but perhaps can assume the latter based on earlier usage.
Here is where things begin to be messed up.
Arpachshad begat Shalach at 35 years
35 / 6 = 5.8!
This is impossible. Note, though, that suddenly there are much shorter lifespans. In fact, about half the lifespan.
I would suggest that at this point, there was a shift in the meaning of shana, and now it meant season. A season is four months, which is twice the previous shana. Thus, each year we would have to divide by 3, rather than 6.
35 / 3 = 11.6
which is the same age as Kenan when he begat his son. (He was purportedly 70.)
Note that this shoft occured about the Shem/Arpachshad time, so perhaps Shem's total age should also be less.
Arpachshad lived 403 (more years), or lived to 146, which is about the same lifespan as preflood people.
And the same for all the people up to Nachor.
We should not think that Terach was 70 (/3 =23) when he had his first child, for we again have an interlude, and all three children are listed:
Perhaps other suggestions later...
I should have mentioned two points:
First, something that might be interpreted to mean that lifespans were being shortened. In Bereishit 6, right before the Mabul:
And furthermore, in the Mabul narrative, there is a mention of the year put against months and days, in Bereishit 7:
Update: For those who feel that this is only speculation -- which it is -- and therefore "improper scholarship", I disagree. But here is some data for comparison, in the Sumerian King list, which I discussed next. That might back it up a bit.