What does Genesis 5:15 mean?

Mahalalel fathered Jared at the relatively spritely age of 65 years old. Jared becomes the sixth in line from Adam. Of the nine patriarchs preceding Noah, Jared will be the last who dies well in advance of the flood. Enoch will be taken by God in some way, prior to his natural death (Genesis 5:24), while Methuselah and Lamech will die immediately prior to the near-total-destruction of mankind.

Along with Enosh and Kenan, Mahalalel and Jared will also live many years concurrently with Noah. It's critical to notice that all four of these men lived hundreds of years in parallel with Adam. In fact, according to this passage, the first patriarch born after Adam's death is Noah himself. The eyewitness testimony of God, and His relationship to mankind, was still alive up to the moment of the flood.

The long life spans in this chapter are attributed to various factors. Cleaner food, water, and air would contribute greatly to longer lives. In addition, human genes had not been as effected by the corruption of the fallen world. Particularly after the flood, life expectancies will drop off dramatically. This change also reflects a deliberate act by God, as explained in Genesis 6:3.