Why did people live so long before the flood?

Adam lived to 930 (Genesis 5:5), Methuselah to 969 (Genesis 5:27), Noah to 950 (Genesis 9:29). Such long ages seem impossible to us today.

Here is a table of ages from Adam to Noah.

(a) Assuming that the relationships are all father to son, which is unlikely.
(b) Enoch did not die, but “God took him” (Gen 5:24). Lifespans reduced rapidly after the flood, although there were still some remarkably long lives. For example, Abraham died at 175 (Genesis 25:7-8). By the time of Moses, the average lifespan was about 70 years.

The years of our life are seventy,
or even by reason of strength eighty;
yet their span is but toil and trouble;
they are soon gone, and we fly away. (Psalm 90:10)

There has been a lot of speculation about what might have caused the average lifespan to reduce so dramatically. In Genesis 6:3, we read

Then the Lord said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.”

Many have taken this to mean that God placed a limit of 120 years on human lifespan that was gradually introduced after the flood. If this is a correct interpretation, it is striking that the oldest person in recent recorded history is Jeanne Calment who died at 122 years and 164 days. However, the sentence could mean that there were 120 years until God would send the flood, in which case it would have no direct relevance to longevity.

There are various physiological explanations that would lead to a reduction in lifespan, but these can only ever be speculative. The Bible provides no clear information about why lifespans reduced, or how it came about.