There are several reasons for the antediluvian lifespans: one had to do with the vast proliferation of pathogens in the world due to the radiation unleashed on the world at the time of the flood; the stress on the human immune system prior to the flood was simply never as great as it is now.
Another had to do with gravity. Gravity is simply the most major form of stress on our bodies, and the same attenuation of gravity which allowed the gigantic animals which lived prior to the flood ( a brachiosaur would simply be crushed by its own weight in our present world) eliminated most of the physical stress which we experience now.
There is no real knowing what the word "year" meant prior to the flood but you don't need to know that to comprehend that antediluvians were biologically different from us as regards lifespans. The natural ratio of maturity to lifespan in our present world is something like 1 - 4; in the antediluvian world in which men were marrying and having first children at 70 and then living to 900, that ratio was more like 1 - 13 or so.
It is a dogma of establishment science that the tale of the biblical flood is a fairytale or, at most, an aggrandized tale of some local or regional flood. That, however, does not jibe with the facts of the historical record. The flood turns out to have been part and parcel of some larger, solar-system-wide calamity.
In particular, the seven days just prior to the flood are mentioned twice within a short space:
Gen. 7:4 "For yet seven days, and I will cause it to rain upon the earth forty days and forty nights;...These were seven days of intense light, generated by some major cosmic event within our system. The Old Testament contains one other reference to these seven days, i.e. Isaiah 30:26:Gen. 7:10 "And it came to pass after seven days, that the waters of the flood were upon the earth."
"...Moreover, the light of the moon shall be as the light of the sun, and the light of the sun shall be sevenfold, as the light of seven days..."
Most interpret this as meaning cramming seven days worth of light into one day. That is wrong; the reference is to the seven days prior to the flood. The reference apparently got translated out of a language which doesn't use articles. It should read "as the light of THE seven days".
It turns out, that the bible claims that Methuselah died in the year of the flood. It may not say so directly, but the ages given in Genesis 5 along with the note that the flood began in the 600'th year of Noah's life (Genesis 7:11) add up that way:
Gen. 5:25 ->"And Methuselah lived an hundred eighty and seven years and begat Lamech. And Methuselah lived after he begat Lamech seven hundred eighty and two years, and begat sons and daughters. And all the days of Methuselah were nine hundred sixty and nine years.
<i.e. he lived 969 - 187 = 782 years after Lamech's birth>
And Lamech lived an hundred eighty and two years and begat a son. And he called his name Noah...
<182 + 600 = 782 also...>
Thus we have Methusaleh dying in the year of the flood; seven days prior to the flood...
Louis Ginzburg's seven-volume "Legends of the Jews", the largest body of Midrashim ever translated into German and English to my knowledge, expands upon the laconic tales of the OT.
From Ginzburg's Legends of the Jews, Vol V, page 175:
...however, Lekah, Gen. 7.4) BR 3.6 (in the week of mourning for Methuselah, God caused the primordial light to shine).... God did not wish Methuselah to die at the same time as the sinners...
The reference is, again, to Gen. 7.4, which reads:
"For yet seven days, and I shall cause it to rain upon the earth forty days and forty nights..."
The note that "God did not wish Methusaleh to die at the same time as the sinners" indicates that Methusaleh died at pretty nearly precisely the beginning of the week prior to the flood. The week of "God causing the primordial lights to shine" was the week of intense light before the flood.
What the old books are actually telling us is that there was a stellar blowout of some sort either close to or within our own system at the time of the flood. The blowout was followed by seven days of intense light and radiation, and then the flood itself. Moreover, the signs of the impending disaster were obvious enough for at least one guy, Noah, to take extraordinary precautions.
The ancient (but historical) world knew a number of seven-day light festivals, Hanukkah, the Roman Saturnalia etc. Velikovsky claimed that all were ultimately derived from the memory of the seven days prior to the flood.
If this entire deal is a made-up story, then here is a case of the storyteller (isaiah) making extra work for himself with no possible benefit, the detail of the seven days of light being supposedly known amongst the population, and never included in the OT story directly.
Greek and Roman authors, particularly Hesiod and ovid, Chinese authors and others, note that small groups of men and animals survived the flood on high places and on anything which could float for a year. I do not see an essential contradiction between this and the biblical account. Noah's descendants were probably unaware of anybody else surviving and wrote the story that way.