Question: Can you explain Methuselah's age?
Answer: Methuselah is the oldest man mentioned in the Bible. He lived to be 969 years of age (Gen. 5: 21-27). In view of the precision of the language and the chronology of events, it is apparent 969 is to be understood as a literal number.
The longevity of antediluvian man. Men before the flood lived much longer than men lived after the flood (notice from Adam to Noah, Gen. 5). The average age of the nine patriarchs (excluding Enoch) was 912 years (Gen. 5). The Bible presents the world before the flood as being a "different world" in a number of important ways, not just man's longevity (Gen. 2-6; 2 Pet. 3: 6).
Something happened to cause man to have a much shorter life-span. It is a Biblical fact that there was a slow decline in life-span from 950 years for Noah to 205 years for Terah (Gen. 11), and down to 70 years when Moses wrote Psalm 90: 10. We need to remember that man was created as an immortal being, but sin brought about death (Rom. 5: 12).
As a matter of fact, we do not know for sure what caused man's life-span to decline. The antediluvian world (before the flood) appears to have had a "different system" to provide water (Gen. 2: 6). One possible explanation is the somatic mutation theory, random change in the structure of a cell in the body. The possible increase in radiation (brought about by probably less resistance to the sun, no "mist") could have caused cellular mutation which could have resulted in accelerated aging and death. However, we are not told.
Beloved, this earth, its atmosphere and man have undergone drastic changes. Some of these changes may well account for some of the apparent contradictions between the Bible and science, generally viewed. A case in point would be how the age of the earth is determined. Rest assured, the Bible is always right.