There's no question -- people in the United States are waiting longer to retire.

Whether it's because they're motivated to stay active in a career, want to make a difference in the world or struggling with skyrocketing bills, 46% of U.S. employees over 50 now say they plan to work longer than they once expected. A third of workers 55 to 64 expect to work at least five years more than originally planned, according to a recent Towers Watson survey.

Here are some top reasons people give for postponing retirement, according to Towers Watson, Boston College's Sloan Center on Aging & Work and other experts on baby boomers and work:

1. They're staying longer in jobs with employer-paid benefits because private health insurance is so expensive. More than two-thirds (68%) of older workers cite health coverage as a reason to delay retirement, according to the May/June 2010 Towers Watson retirement attitudes survey of 3,099 U.S. workers in nongovernment jobs. 

Covering health-care costs is even more important to people in poor health: Of that group, an overwhelming majority (77%) say they'll keep working until they're eligible for Medicare benefits.

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2. They're saddled with debt. Older Americans are sinking under credit card, medical and other bills; 63% of workers said they were actively paying off debt, more than double the number from 2009, according to the Towers Watson survey. Retire Smart columnist Mark Miller found that in a recent national sampling of consumer debtors, the average age of bankruptcy petitioners is rising, with the sharpest increase among those over age 55. 

While older people account for a relatively small share of overall bankruptcy filings, the rates of increase are dramatic: From 1991 to 2007, the percentage of bankruptcy petitioners age 65 to 74 rose 178%.

3. Their homes are their main retirement savings, but they're mortgaged to the hilt or can't sell in the current market. Some boomers dream of moving someplace new and affordable in retirement. But realizing that dream is getting harder. According to Miller, 63% of people in their late 50s and early 60s are carrying mortgage and home-equity debt, up from 49% in 1989. Since the housing bubble burst, it's gotten a lot harder to sell. 

Between 2005 and 2010, mobility rates among older U.S. homeowners fell 39%, the sharpest drop of all age groups, according to a report from Harvard University's Joint Center for Housing Studies, which predicts some homeowners will not be able to retire elsewhere unless housing and financial markets stage a sharp rebound.

4. They tapped into 401(k)s and other retirement savings accounts during the recession. Fidelity Investments reported that among its active 401(k) plan participants 11% borrowed or withdrew funds from their accounts during the year ending in June 2010. That's a 10-year high.

5. They worry disappearing pensions will make it harder to stop working. Financial planning experts talk about the three-legged stool that supports retirement security: Social Security, individual retirement savings and pensions. 

But all three legs look wobbly, including defined benefit pensions, which have evaporated in the private sector as more companies shift to defined contribution programs, mainly 401(k) plans, Miller notes in his column. According to research commissioned by the National Institute on Retirement Security, 77% of people believe the disappearance of pensions has made it harder to achieve the American Dream.

6. They're caring for ailing parents, putting kids through college -- or both. Sandwich Generation boomers are grappling with how to help aging parents who can't or don't want to stay in their homes, but want alternatives to hospital-like care facilities.

Meanwhile, some boomers are tapping into retirement savings to pay for their children's college education, which hit $26,273 for a year at a private school in 2009-2010, according to the College Board and SecondAct personal finance columnist Karin Price Mueller. That's up 4.4% from the previous year, slightly under the 6.5% increase in a year's worth of tuition and fees at a four-year public institution, which hit $7,020 for the 2009-2010 school year, not including room and board.

7. They haven't had "the talk." It's common for couples, even if they've been together for many years, to put off talking about retirement topics, including when to stop working and where they want to live once they do. Dorian Mintzer, a Boston psychologist and career/life transition coach, suggests these 10 discussion questions to help couples devise a mutually agreeable retirement plan and timetable. 

"With dual-career couples and longer life spans, there are many more options -- retiring together or separately, phasing out and working part time, using your skills for an encore career, volunteering, taking care of grandchildren or caring for an elderly relative," Mintzer says.

8. They're finding new opportunities in a job market that's improving, including for older workers. During April, the unemployment rate for employees over 45 fell to 6.6%, the lowest it's been since May 2009, according to the Bureau of Labor Statistics. 

As the economy improves, the need for experienced professionals is greater than ever, says John Fulcher, director of MRINetwork's Bauer Consulting Group Inc., an El Paso health-care recruiter. "We are seeing the need for more seasoned professionals who require less ramp-up time and are able to make a larger impact in a shorter amount of time," Fulcher says.

9. They prefer to keep working as long as they're physically able. Boston College researchers note that one in five workers 50 and older has retired from a career job but currently works for pay in a new role -- what the researchers call a "retirement job" and others term a "bridge job." 

In the future, according to separate research from the college and the Families and Work Institute (FWI), 75% of people 50 and over expect to work in some type of second act job.

10. They want to make a difference. Aside from financial reasons, many people keep working because they want to stay active or "contribute and be productive," researchers at Boston College and FWI found. More people are using the second half of life to take on second careers, start businesses or follow long-delayed dreams to serve their communities, social entrepreneurship pioneer and Civic Ventures CEO Marc Freedman writes his new book, The Big Shift. 

"It's a time," he writes, "when many have insight about what matters, a special impetus to act on this wisdom, and the ability to do so. In this respect, it's a potential sweet spot, a confluence rather than a reinvention."

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