Medical advances and living standards have extended the average human longevity from 48 years in 1955 to 71 years today, and the elderly are now the fastest growing segment of society. But while our life spans are improving, our health spans are not, writes science journalist Sue Armstrong in “Borrowed Time: The Science of How and Why We Age” (Bloomsbury), out now.

“Over the past 50 years, health care hasn’t slowed the aging process so much as it has slowed the dying process,” she writes, quoting gerontologist Eileen Crimmins.

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