Read this article at its new location here: https://reformedwiki.com/did-people-live-hundreds-of-years-in-genesis
Adam, as the first human created and as one created without sin, likely had such an uncorrupted body and genetic constitution that he, and his descendants after him for some time, was able to live hundreds of years.
Some argue that it is completely unrealistic and impossible to believe that humans in Genesis could have lived hundreds of years. For example, Genesis teaches that Adam lived 930 years, Seth lived 912 years, and Methuselah lived 969 years:
Thus all the days that Adam lived were 930 years, and he died. (Genesis 5:5
Thus all the days of Seth were 912 years, and he died. (Genesis 5:8
Thus all the days of Methuselah were 969 years, and he died. (Genesis 5:27
People today live nowhere near that long, so how could any human possibly live that long?
We cannot know for certain exactly why or how people lived for hundreds of years in Genesis. However, there is a very plausible explanation that would answer objections that it is completely unrealistic and impossible to believe something like this.
Adam was created without sin, which means that his body, including his genetics, would have been in perfect condition. It is very possible that his body would have been in such great condition that he, and his descendents after him for some time, would have been able to live for hundreds of years.
However, as mankind continued to procreate, sin continued to corrupt the bodies of humans, so that they lived for shorter and shorter periods of time.
Humans living for a shorter period of time later in history is connected with God's judgment upon humankind for its sin, and the floor, which God uses to punish humans. In Genesis 6:3
My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years. (Genesis 6:3
Then, immediately after this, Genesis 6:5-7
The Lord saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually. And the Lord regretted that he had made man on the earth, and it grieved him to his heart. So the Lord said, "I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens, for I am sorry that I have made them."
So, it is possible that humans being limited to living 120 years is either the direct result of God's judgment and decree, or perhaps it is the result of something that happened through the flood. It is possible that the flood changed something in earth's environment that would cause humans to live significantly shorter lives.
Anything that the Bible does not explicitly teach is merely conjecture and not certainty. However, what is important is that there are plausible explanations that make the fact that some people lived for hundreds of years in Genesis something that is very possible to believe.
There are very plausible reasons to believe that people in Genesis could have lived hundreds of years, and no reason to believe that it would have been impossible for this to happen. We can believe what the Bible teaches without compromising rationality.
Some people have trouble accepting possible explanations to alleged contradictions in the Bible, especially ones that they consider a stretch. For example, Dan Barker, president of the Freedom From Religion Foundation, says this:
I have received numerous replies from Christians who think that these contradictions are either trivial or easily explained. Yet not a single "explanation" has been convincing.
The fact is that there are plausible explanations for every alleged contradiction in the Bible. That there is a plausible explanation for an alleged contradiction does not mean that it is definitely the correct explanation for the alleged contradiction.
However, as long as a possible explanation has been suggested, then it has been objectively demonstrated that there is no necessary contradiction regarding the Bible verses and passages brought up.
When people like Dan Barker say that they don't find a particular explanation for a contradiction "convincing," then that is merely their opinion. A plausible explanation has been suggested that eliminates the necessary alleged contradiction. They simply don't like it, which is not at all a relevant argument against the explanation.
To read more about solutions to Bible contradictions and difficulties, check out Norman Geisler's The Big Book of Bible Difficulties: Clear and Concise Answers from Genesis to Revelation. While we do not agree with some of Geisler's theology, particularly concerning his view of predestination, this book is still an excellent resource. It is thorough and filled with research.
Another book to check out is Tim Chaffey's Demolishing Supposed Bible Contradictions: Exploring Forty Alleged Contraditions, which also answers many alleged contradictions in the Bible.