The reducing ages may be meant to suggest the cumulative effects of sin

Genesis 5:1-32
Adam lived were 930 years.
Enoch walked with God after he fathered Methuselah 300 years and had other sons and daughters. Thus all of the days of Enoch were 365 years.
All days of Methuselah were 969 years, the most high ages.
Methyselah, he fathered Lamech. Lamech fathered a son Noah. Noah fathered Shem, Ham and Japheth.
Why?
The Bible often provides family trees similar to this one to define an important line of descent. Many of them are selective, sometimes is order to create a pattern of a certain number of names. We are not meant to add up all the figures and work out the length of time covered. (Ussher did this, arriving at 4004 BC for the date of Adam. Yet we know from Jericho and other cities, of a civilization going back to 7000BC, and that is not the  beginning of human history.)
In the ancient Near East numbers were often used to signify importance, rather than actual quantity. The reducing ages from Methuselah to Joseph may be meant to suggest the cumulative effects of sin.
Enoch's life of 365 years, a complete year's cycle of days, is worth special note.
The life-span assigned to these early ancestors is remarkable. It ranges from 777 years for Lamech to Methusleah's 969 years. Each of the ten records follows the same formula: When A was x years old he had a son B. He lived another y years and had other sons and daughters. He lived z years, and them he died.
How?
Most high ages of man now is about 120 years. The reducing ages may be meant to suggest the cumulative effects of sin