It is somewhat of a mystery why people in early chapters of Genesis lived such long lives. There are many theories put forward by biblical scholars. The genealogy in Genesis 5 records the line of the godly descendants of Seth—the line that would eventually produce the Messiah. God possibly blessed this line with especially long life as a result of their godliness and obedience. While this is a possible explanation, the Bible nowhere specifically limits the long lifespans to the individuals mentioned in Genesis chapter 5. Further, other than Enoch, Genesis 5 does not identify any of the individuals as being especially godly. It is likely that everyone during that time lived several hundred years. Several factors may have contributed to this.
Something happened at the global flood to shorten men’s lifespans. Compare the lifespans before the flood (Genesis 5:1–32) with those after the flood (Genesis 11:10–32). Immediately after the flood, the ages decreased dramatically and then kept decreasing. A key may be in Genesis 6:3: “The Lord said, ‘My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.’” Many people see the reference to “a hundred and twenty years” as the new, divinely appointed limit on man’s age. By the time of Moses (who lived 120 years), lifespans were much lower. After Moses, only one person is recorded as having lived past 120 (2 Chronicles 24:15).
One theory for why the people of Genesis lived such long lives is based on the idea that a canopy of water used to surround the earth. According to the canopy theory, the water “above the firmament” (Genesis 1:7, KJV) created a greenhouse effect and blocked much of the radiation that now hits the earth, resulting in ideal living conditions. At the time of the flood, the water canopy was poured out on the earth (Genesis 7:11), ending the ideal environment. The canopy theory has been abandoned by most creationists today.
Another consideration is that, in the first few generations after creation, the human genetic code had developed few defects. Adam and Eve were created perfect. They were surely highly resistant to disease and illness. Their descendants would have inherited these advantages, albeit to lesser degrees. Over time, as a result of sin, the human genetic code became increasingly corrupted, and human beings became more and more susceptible to death and disease. This would also have resulted in drastically reduced lifespans.