What does Genesis 5:20 mean?

The ages at death of those listed in this book of Adam, so far, are as follows: Adam-930, Seth-912, Enosh-905, Kenan-910, Mahalalel-895, and Jared-962. Whether by coincidence or by design, Jared's life is the longest (so far) in Genesis, while his son Enoch's is the shortest, at 365. However, Jared's record for longevity won't stand. His grandson, Methuselah, will become the go-to punchline for old age, living to 965 years.

Just prior to the flood, God will make a deliberate decision to drastically limit human lifespans (Genesis 6:3). This is probably meant to limit the amount of evil man is capable of. With only a few decades of active life, rather than several centuries, it's much harder for one man to gain the power required to commit wide-spread carnage. Sadly, as history has shown, this abuse is merely harder, not impossible.

The phrase "and he died" is repeated for every one of the patriarchs leading to Noah, other than Enoch (Genesis 5:24). This is the most immediate effect of the fall: physical death. This was the main consequence God gave to Adam (Genesis 2:17) regarding the Tree of the Knowledge of Good and Evil, and it is repeated over and over here, for that very reason.