Gen 6:3
And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
"Man" & "his" doesn't describe Sarah.
Man describes humans, with all their variety. There is a subset of humans, who have an extra layer of the divine, beyond just average human. In the case of Sarah, her divine side gave her some extra life since she was human+. Before then, human++ was more common.
The analogy is the entrepreneur who changes the world. Their children, who are born to entitlement, spend too much time being shallow and pretentious in the country club party circuit. Burning the candle at both ends, has a long term impact on their health and attitudes; human-. Humans were responsible for the lowering of their own life expectancy. But every now and then a new person appears with that divinely boosted spirit; Sarah and human+.
Science behind long life
Neurons, as a rule, do not replicate very often. Many of our neurons are as old as we are. Some neuron can and do replicate, but this is not as common as old neurons. The brain beside supporting consciousness, also plays a role in the body's cellular differentiation control system, for all other cells. As we age, the brain and most of our old neurons do not recover, like other cells, since cellular replication is not as common for that solution. The entire cellular control system starts to break down; old age causes other cells to deviate.
Back in the ancient days, the stories seem to suggest that neurons could and did replicate more often. If this had occurred, in a sequential way, it would allow a continuity of memory for consciousness, as well as a longer life, due to refreshing the cellular differentiation control system.
The human brain has two centers of consciousness; inner self and ego. The willpower of the ego is a large energy draw from the brain; burning the candle from both ends. This discourages neuron replication. Mother cells. to make two daughter cells, will needs to store sufficient food and energy. However the ego, by its extra energy draw, causes more firing of the neurons, beyond natural. This causing many neurons to eat the stockpile of food needed for replication. They are stuck unable to replicate.
By the time of Sarah, the human ego had become very strong and had a sizable energy draw, that limited life. It ate away at the neuron food stockpiles for replication. Sarah was more selfless; weaker ego, so she had some extra neuron replication for her human plus.
Science suggest that aging is connected to the telomere aspect of chromosomes. These are located at the end of chromosomes and shorten each replication cycle. After enough shortening of the telomere, duplication of the DNA gets more compromised.
Neuron do not have this telomere problem, since they rarely replicate. Neurons conserved their telomere so they telomere have the potential to last much longer, than the rest of the cells of the body. They can be used by the neurons to help to refresh other cells, via the control system. But neurons are extremely energy intensive; 90% energy used maintaining the membrane potential. This makes it harder for them to replicate for the needs of longer life via cellular differentiation control.