the only thing i can think of is that Gods decrees didnt always take place immediately
for example, it was 100 years before the flood arrived after it being decreed. In Gen 5, we see the generations of Adam - Noah, and Noah was 500 years old.
then in Gen 6 we get the decree that God would limit mans age, then he also decrees that the deluge would come. The deluge was said to come in the 600th year of Noahs life. Thats a gap of 100 years from the time of the decree to the time of the event
So, it just shows that a decree does not have to take effect immediately. I think you will find that after the sons of Noah began to have children, those life spans were somewhere around the 120 year old mark ... Sarah for instance considered herself to be of 'old age' at the age of 127 as you mention
she probably died not long after that
Its also noteworthy to think that, before the flood, humans probably didnt have the disease and illnesses and we have today...many of which are passed along genetically. This could have contributed to their longevity
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