Bankruptcy rates among seniors have more than doubled since the early 1990s, amid rising health-care costs and mounting personal debt, new research shows.

Of more than one million Americans who filed for bankruptcy last year, seniors accounted for about 22.3 percent, up from just 8.2 percent in 1991, according to a study released this week by AARP's Public Policy Institute.

Over the same period, the number of younger bankruptcy filers declined. Last year, about a quarter of all bankruptcy filers were under 34 years old, down from roughly half in 1991, the study found. Overall, the median age of bankruptcy filers rose over the past decade from 36 to 43. 

Researchers blamed the higher rates among seniors on rising health-care costs, a declining economy and a general lack of financial planning.

"Our culture has normalized debt," Elizabeth Warren, a Harvard Law professor who conducted the study, said in a statement. "Now, individuals nearing or in retirement are realizing how difficult it can be to manage that debt as they age," she said.