What adjective would you use to describe someone who just turned 60? It probably wouldn't be "elderly" -- unless you're the federal government. When Congress set up the still-functioning Tax Counseling for the Elderly program in 1978, it defined an "elderly individual" as a person "who has attained the age of 60."

But 40 years ago, many did consider a 60-year-old to be elderly -- including the legion of then twenty-something boomers who today bristle at being called old, let alone elderly. These days, where "60 is the new 50" and many people enjoy robust health into their 80s and beyond, almost no one believes 60, or even 65, is "elderly." Except for government policymakers.

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"The standard measures of aging have people getting old at 65, and they were always getting old at 65," says Warren Sanderson, a professor of economics at Stony Brook University. "They got old at 65 in 1930, they get old at 65 in 2015, and the forecast says they're going to get old at 65, 50 years from now."

Sanderson has published new research showing that although there are more older adults than ever, population aging is actually slowing because of increased life expectancy. But because governments fail to recognize these changing dynamics, they create flawed polices. He says instead of using 65 as an almost mystical age when a person immediately becomes old, a better metric is "time left until death." In 1940, a 65-year-old man could expect to live another 12.7 years. A man turning 65 today can expect to live another 19.3 years. That pales in comparison when you consider recent studies have suggested that most babies born today will live to be 100. "We're not going to be retiring at age 65 or 66 or 67 if we're going to be living to 100," Sanderson says.

Sanderson's research has implications for Social Security. "The Social Security Trust Fund is going to run out of money somewhere between 2030 and 2033," Sanderson says. One way to ensure Social Security's solvency is to raise the age for full benefits, something Congress first did in 1983 when it mandated the gradual raising of full retirement age from 65 to 67 over a 22-year period beginning in 2003. Because the first workers facing the higher retirement age were only 45 when the law was passed, it gave them time to prepare. "What would happen if we simply said, 'Let's just continue this?'" Sanderson says. "The longer we delay it, the more likely we'll do something that is bad for older folks.