At around age 60, "muscles really start to break down," say Kathryn Starr, an aging researcher, "and because of that, the protein needs of an older adult actually increase."

在60岁左右，“肌肉真的开始分解，”一位研究衰老的研究员凯瑟琳·斯塔尔说，“也正因为如此，老年人对蛋白质的需求实际上增加了。”

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One possibility relates to the ATP-making machinery's emission of free radicals, which are thought to contribute to aging and to such age-related diseases as cancer by damaging cells.

一个可能的解释与制造 ATP 的组织释放出的自由基有关，人们认为它通过破坏细胞来促进老化，并且导致这种像癌症一样的和年龄有关的疾病。

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The energy powerhouse cells in muscles get damaged with age. That damage accumulates over time and, on top of that, the body's ability to repair that damage also dwindles with aging.

瓦那嘎特说，就像是连挨了两拳，变老以后，肌肉里的能量单元不断损耗，随着时间的推移，这种伤害不断积累；除此以外，身体对于损害的修复能力也因老化而缩小。

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And entire industries have grown up around the idea of productive aging, the third age, or all the other marketing euphemisms out there.

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After all, in the spring of 1997, when Microsoft announced the acquisition, WebTV Networks was a two-year-old stripling of a startup with an oversized marketing budget, an aging demographic (25% over the age of 50) and a scant number of subscribers (less than 100, 000).

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An analysis of data from the past three censuses shows that because of baby boomers' "aging in place, " the population of those age 45 and over grew 18 times as fast as the population under age 45 between 2000 and 2010, said William Frey, a demographer and senior fellow in the Metropolitan Policy Program at the Brookings Institute.

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