According to the Bible, people once lived for nearly a thousand years. Adam lived 930 years. Methuselah lived the longest of any person mentioned, dying at 969 years of age (see Genesis 5), and Noah nearly equaled him at 960 years (Genesis 9:28-29).
Shortly after the Flood, however, we find people living much shorter life spans. Abraham, for example, lived 175 years (Genesis 25:7). A few generations later, Joseph lived "only" 110 years (Genesis 50:22). We are told in the Psalms that 70 years is considered to be a long life, and men reach 80 years only "by reason of strength" (Psalm 90:10). David died at age 70 in about 1000 BC and is described as dying "in a good old age, full of days" (I Chronicles 29:28).
There have been many speculations concerning the great length of human life spans before the Flood, such as the earth having a thick water vapor canopy that blocked out most of the sun's harmful radiation, or double atmospheric pressure that enhanced the health of all organic life. The Bible, however, does not directly explain why this was or why life spans shortened after the Flood.
Possibly, God simply determined that the longer pre-Flood life spans only multiplied the evil men can do (see Genesis 6:3-7, 11-13) and that about 70-120 years is long enough for a person to fulfill his purpose. Those who are called can prepare in that length of time to receive immortality at the return of Christ. Those not yet called can learn that the ways of man do not bring satisfaction or fulfillment. Thus, when they arise in the second resurrection, they will be prepared to choose God's way.