What does Genesis 5:8 mean?

Seth died at 912 years old, living a slightly shorter life than his father, Adam. Despite living more than nine centuries, Seth did eventually die. In fact, a noteworthy aspect of this chapter is that, in spite of their long lives, we're repeatedly reminded that each of these men died. Sin had brought death to the world, and death always won. The only apparent exception is that of Enoch, as described in Genesis 5:24.

We'll also see that human lifespans will begin to decrease. Death will come sooner as the generations get farther removed from Adam, especially following the flood. This is one reason that many scholars attribute the long lives of early man to a pure environment and brand-new, uncorrupted genetics. Over time, as the effects of the fall continued to work, both the biology of man, and the environment of earth, would make it less and less likely for a man to live beyond a few years.

The long lives shown in this chapter also serve to maintain the value of an oral history. Having eyewitnesses survive for centuries and multiple generations, would have made corruption of history all but impossible.