Hi CG, there are no actual doctrines on this point, but the Church says that we are to interpret the Scriptures literally unless the literal interpretation is “untenable”. For example, it would be untenable to interpret literally God’s “hands” since God is Spirit. But if God wanted humans to live for more than 900 years, and then later reduce our life spans then that obviously is not untenable. Here is the great Father and Doctor, St. Augustine on this point:
"Wherefore no one who considerately weighs facts will doubt that Cain might have built a city, and that a large one, when it is observed how prolonged were the lives of men, unless perhaps some sceptic take exception to this very length of years which our authors ascribe to the antediluvians and deny that this is credible… But the length of an antediluvian’s life cannot now be proved by any such monumental evidence. But we are not on this account to withhold our faith from the sacred history, whose statements of past fact we are the more inexcusable in discrediting, as we see the accuracy of its prediction of what was future…
For they are by no means to be listened to who suppose that in those times years were differently reckoned, and were so short that one of our years may be supposed to be equal to ten of theirs. So that they say, when we read or hear that some man lived 900 years, we should understand ninety, ten of those years making but one of ours, and ten of ours equalling 100 of theirs…
By these plausible arguments certain persons, with no desire to weaken the credit of this sacred history, but rather to facilitate belief in it by removing the difficulty of such incredible longevity, have been themselves persuaded, and think they act wisely in persuading others, that in these days the year was so brief that ten of their years equal but one of ours, while ten of ours equal 100 of theirs. But there is the plainest evidence to show that this is quite false…
It is proved by this, that Scripture states that the flood occurred in the six hundredth year of Noah’s life. But why in the same place is it also written, “The waters of the flood were upon the earth in the six hundredth year of Noah’s life, in the second month, the twenty-seventh day of the month,” if that very brief year (of which it took ten to make one of ours) consisted of thirty-six days? For so scant a year, if the ancient usage dignified it with the name of year, either has not months, or this month must be three days, so that it may have twelve of them… Who can away with such foolishness and absurdity? Far be this error from us—an error which seeks to build up our faith in the divine Scriptures on false conjecture only to demolish our faith at another point. It is plain that the day then was what it now is, a space of four-and-twenty hours, determined by the lapse of day and night; the month then equal to the month now, which is defined by the rise and completion of one moon; the year then equal to the year now, which is completed by twelve lunar months, with the addition of five days and a fourth to adjust it with the course of the sun. It was a year of this length which was reckoned the six hundredth of Noah’s life, and in the second month, the twenty-seventh day of the month, the flood began—a flood which, as is recorded, was caused by heavy rains continuing for forty days, which days had not only two hours and a little more, but four-and-twenty hours, completing a night and a day. And consequently those antediluvians lived more than 900 years, which were years as long as those which afterwards Abraham lived 175 of, and after him his son Isaac 180, and his son Jacob nearly 150, and some time after, Moses 120, and men now seventy or eighty, or not much longer, of which years it is said, “their strength is labor and sorrow.”" (The City of God, XV, 9-14)
