For some, they simply love what they do. For many more, there are just too many bills to pay.

In huge numbers, members of the baby boom generation — born from 1946 through 1964 — tell researchers that they don’t plan to retire. In one recent AARP survey, nearly 70 percent of baby boomers reported they intend to work past the traditional retirement age of 65.

Those numbers have given rise to a fair amount of happy talk about how this generation is poised to reinvent retirement. Yet the retirement picture, like so much else for the nation’s 78 million graying baby boomers, is complex.

On the one hand, baby boomers like to work: Despite a generational stereotype portraying them as free spirits who reject tradition, boomers in the prime of their working years have enthusiastically embraced the work ethic, often defining themselves by their careers.

Throwing on the career brakes at 65 simply sounds counterintuitive to many baby boomers. The working world has long embraced them, largely because they’re better educated than the generations that came before or since, with almost 30 percent holding at least a bachelor’s degree and another 30 percent having attended college.

But it’s also true that with the death of traditional company pensions and more recently the biggest economic downturn since the Great Depression, the baby boom generation in many ways has no choice but to redefine what retirement means.

Many would leave the daily grind of jobs if they could. But continuing financial obligations to aging parents and grown children, as well as financial burdens left by the recession, have combined to make many boomers’ retirement prospects more difficult.

“The reason that older participation in the workforce increased has nothing to do with the health and well-being of people that age,” said social critic Susan Jacoby, author of “Never Say Die: The Myth and Marketing of the New Old Age.”

She added: “It’s an economic need.”

Working boomers on the rise

The average retirement age in the United States hit a low of 62 in the mid-1990s, when the majority of boomers’ parents were retiring; today, it is 64 and climbing, according to the Bureau of Labor Statistics.

Seniors’ portion of the workforce has risen, as well. By 2020, according to the bureau’s figures, baby boomers will account for more than 25 percent of the workforce, up from 16 percent today.

That figure rivals the share of older adults who continued working past age 65 in 1951, before Social Security was fully phased in for all professions and people could count on retirement income.

In short, what most baby boomers face is far from their parents’ version of retirement, which began early — at age 55, for many — and has lasted for many decades’ worth of bridge games and golf excursions.

For public sector employees whose benefits include a defined monthly retirement income, that sort of retirement remains possible. But across the country, public employee ranks are thinning, and public pension reforms are beginning to reshape their retirement landscape, too.

“Benefits for retirement have been declining in the private sector,” said Chris Hoene, executive director of the California Budget Project, “and now the public sector is following suit.”

For millions of private sector workers nearing retirement, the shift in their economic future has already occurred.

The nation’s three-decade transition away from defined benefit pensions and company-funded retiree health benefits has stripped from millions of boomer-aged Americans the kind of retirement security their parents took for granted.

“Everyone I know is terrified of outliving their money,” said AARP’s national jobs expert Kerry Hannon, who blogs for Forbes on retirement issues. “Most people are going to need to work in some fashion past what we think of as retirement age, and that will primarily be a financial decision.”

Familiar financial issues

In 1980, 80 percent of Americans working in the private sector relied on companies’ traditional defined benefit plans as the foundation of their retirement finances, according to Employee Benefit Research Institute (EBRI) figures.

Then private companies latched onto tax-deferred 401(k) plans — the retirement savings accounts originally created to help executives shelter extra money in addition to their pensions — as a way to shed the burden of providing benefits to retirees.

By 2005, EBRI says, only one-third of private sector workers had either traditional pensions or a combination of a company pension plus 401(k).

Half of all working Americans have no retirement plans in place at all.

In a sense, millions of baby boomers have helped test whether the 401(k) is a reliable financial model for retirement.

The answer? Most people haven’t saved enough to make it work.

Experts say the average 401(k) balance now is about $75,000: Stock market gains in recent months have helped offset the fact that up to one-fourth of people tapped into their retirement savings during the tough recession years.

Even so, EBRI estimates that retirees will need an average 401(k) balance of $900,000 to support them in the extended longevity of their old age.

For boomers hit hard by the recession, continued employment is the only option.

“There are a lot of aging baby boomers who want to find work and can’t,” said Jacoby. “When you look at the economic losses of the past five years, you realize that the baby boomers who still have jobs will have to be carried out feet first.”