In his book about the history of the Jews, the first century historian Josephus stated the following regarding the advanced ages of the pre-flood patriarchs.
"But let no one, upon comparing the lives of the ancients with our lives, and with the few years which we now live, think that what we have said of them is false . . . " (Antiquities (History) of the Jews, Book 1, Chapter 3).
Josephus goes on to states that the reason for the very long lifespans of the pre-flood people was that God decided to be merciful to them and that the food they ate was much better for their bodies than the food eaten post-deluge. The average life span before the flood (ten generations) was 857 years. After the flood, however, the average life span (for the ten generations from Shem to Abraham) was only 317 years! In 2008 the average life expectancy of a person living in the United States was 78 years.