Was there a different method of counting of age at the time of events of Old Testament? (Catholic perspective)

The ancient Jews, including Moses, documented time differently than we do today. The Jewish year was based on lunar cycles (29.5 days), but a year was/is about 12.4 lunar cycles long. The Jews compensated for this by occasionally adding in an extra month, so the years were sort of shorter, but not enough to account for the incredibly long lives of the pre-flood peoples. (See more about the Jewish calendar here) So, I don't think there is any evidence of a severely shortened method of counting years, to answer your question.

However, the interpretation of what a year actually is has been questioned, some assuming a year is not to be taken literally. Even the personhood of the patriarchs has been questioned, some proposing that the patriarchs are actually eras of rule, or named nations.

Consider the following:

"What is the literal sense of a passage is not always as obvious in the speeches and writings of the ancient authors of the East, as it is in the works of our own time. For what they wished to express is not to be determined by the rules of grammar and philology alone, nor solely by the context; the interpreter must, as it were, go back wholly in spirit to those remote centuries of the East and with the aid of history, archaeology, ethnology, and other sciences, accurately determine what modes of writing, so to speak, the authors of that ancient period would be likely to use, and in fact did use. For the ancient peoples of the East, in order to express their ideas, did not always employ those forms or kinds of speech which we use today; but rather those used by the men of their times and countries. What those exactly were the commentator cannot determine as it were in advance, but only after a careful examination of the ancient literature of the East" (Divino Afflante Spiritu 35–36). -Pope Pius XII

Consider that the earth might have been very different preceding a worldwide deluge. We can't know what those conditions were, nor can we know how they impacted the longevity of human life. I've heard many theories on this, but I hesitate to discuss them here because I don't know how well-based they are on scientific evidence.

In addition, during those early days of humanity, the aging process might well have functioned very differently. The theories based upon the evolutionary process within our genetic history are very interesting. The late age at which these historical figures reproduced is notable--indicating a very different reproductive approach from what we know today. Many scientists in the field of study associated with the aging process see a strong tie between longevity and late reproduction.

Consider also that viruses and bacteria like we have today didn't exist in their current form during those days. Viral infections forever alter our DNA, and our children's DNA. That accumulates and alters us, potentially impacting our top end lifespan.

It could be a complex collection of influences, in conjunction with God's own will. It is an interesting field of study and, if these lifespans are really true, might contain some secrets we have yet to determine about how to prolong our lives today.

There is much out there that goes into depth with theories, and some are scripturally backed up. These are all theories with many suppositions and many real scientific objections. Exercise caution as you study.

Good luck!

As far as "from a Catholic perspective", I had a similar question when going through school. It was answered by my professor, a priest of the Basilian Order, in a two-fold manner. He stated that, when it comes to the Old Testament, the Church is happy to allow its followers to believe it is "historically accurate" or "stories told to tell a moral Truth and lay the groundwork for the coming of Jesus Christ." In this way, one must decide to take the numbers given as literal, or given as a moral example to their holiness. The other point made was that in the Hebrew language, characters are both letters and numbers. In this manner, certain numbers may be equal to certain words and the other way around. This is something which the Western mind can have trouble grasping. An example being the Hebrew word for "having completed" or "completeness" equaling the number 40. So, when the People of Israel wandered the desert for 40 years, did they truly wander that long, or until they had completed their imposed tasks by God for their actions at Mount Sinai? I find it to be an interesting notion to contemplate! And as far as from a Catholic perspective, both are correct. This could apply to ages as well. So, did they live that long, literally? Does the longer they lived represent their being Holy and in fellowship with God? Or could these ages be numbers akin to a word, descriptive of the person to whom the age is attached, that would have been easily recognized by those using the ancient Hebrew language? Each of these could be "True" in the sense that, no matter which you use, they are morally true in the context of the Books and stories they are found and in laying groundwork for the coming of Jesus Christ. Since then, I have kept this in mind whenever examining such things as age in the Old Testament and I have made sure to put this before the minds and hearts of those I have taught or conversed with concerning this topic.