Good news, boomers. It really is true: 65 is the new 55.

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Demographers in Austria have constructed what they call "a new toolbox" to study aging. Their chief finding: True age is not simply the number of years a person has lived. Because life spans are increasing and people these days tend to be more active as they hit their 60s than past generations, a person's true age should take into account health and cognitive function. Researchers said that 40 or 50 years ago, a person 65 was considered old. But someone 65 today is probably closer in health and activity levels to a 55-year-old of the 1950s and '60s.

Researchers said changing society's conception of who is old will lead to "a much richer and more realistic view" of aging. The study was published in the journal Population and Development Review.