NEW YORK (CNN/Money) - Last week, Fed chairman Alan Greenspan warned that the number of retirees will soon grow so large that it could threaten the nation's ability to fund Social Security and Medicare.

By 2030, the Census Bureau projects, the population of Americans over age 65 will exceed 70 million, about twice the current number.

So does the burgeoning senior population spell inevitable doom? Maybe not.

The reason is that baby boomers aren't approaching retirement age with an eye toward slowing down. Instead, many boomers intend to remain active, engaged, and gainfully employed. Many who don't have to work, will prefer to work.

"I don't think vibrant and successful people will just sit down and retire," says Susan Ascher, president and CEO of the Ascher Group, a human resources contract staffing firm. "They have a lot of energy and creativity – they're not going to play golf seven days a week."

If 60 becomes the new 40, then, America's 77 million boomers may embrace an oxymoronic concept: the working retirement.

The whole concept of retirement is a relatively recent phenomenon, points out Bob Morison, executive VP of the Concours Group, a management consulting firm in Kingwood, Tex.

"It was institutionalized during the Depression, interrupted a bit by World War II, and hardened after the soldiers came home needing jobs," he says.

Older workers Millions of Americans continue to work after age 60 Age groups  Number working  Percentage of demographic  65 plus  4.8 million  14 percent  60 to 64  6.1 million  51 percent   Source:  Bureau of Labor Statistics

Labor-force participation among Americans over age 65 declined steadilly from 1950 through 1985, falling from 26.7 percent of the population to 10.8 percent, according to Sara Rix, senior policy adviser for AARP.

By 2003 more than 14 percent of 65 and older Americans were employed. The "era of ever-earlier retirement has ended," Rix says.

Concours Group recently surveyed 8,000 full-time workers about their retirement plans. A third of the respondents wanted to keep working full time after age 65, and a third would work part time. The others would quit right away.

Many successful managers return to work as consultants or contract workers, and find they enjoy the greater flexibility. They work at their own rate and intensity, and with less managerial responsibility. "They like their colleagues, they like the work," say Morison. "But they love no longer having to write weekly reports."

The survey's respondents said they would work later in life to make money, of course. But they also want to interact with others. "Working exposes them to social contacts," says Ascher.

Making a contribution to society is also a factor. "The prime reason for people to work after retirement is the need to matter," says Nancy Schlossberg, a counseling psychologist specializing in transitions. "Just as you need financial equity, you need social equity."

Leland Bradford, a co-founder of the National Training Laboratory, envisioned a rich, "elder-statesman" type of retirement. Instead, according to Schlossberg, he got an "About Schmidt" one. Like Jack Nicholson in the movie, he found that ex-colleagues only interacted with him out of a sense of duty. "Bradford moved away," she says, "because he couldn't stand feeling irrelevant."

In her book, "Retire Smart, Retire Happy: Finding Your True Path In Life," Schlossberg identified several different personality types to a working retirement. They include:

• Continuers, which Schlossberg describes as people like herself, who continue to work in their fields, although sometimes at a slower pace. She no longer teaches classes at the University of Maryland, but she writes and consults.
• Adventurers, who find a whole new second career in an unrelated field. One man, in his late 50s, left a research arm of Congress. Finding work was difficult, so he became a massage therapist. All his friends, briefcase lawyer types, were shocked, but he felt really good about the change.
• Searchers are retirees having trouble finding meaningful jobs so they keep switching fields until they do.

All three of these types can find joy and fulfillment in retirement, as can two of her other paths, Involved Spectators and Easy Gliders. Neither of these work, but both remain engaged by keeping on top of events in their fields and the world at large (involved spectators) or through satisfying social activities or hobbies (easy gliders).

"The only negative path is the one taken by Retreaters," says Schlossberg. Retreaters can't find a niche after they leave work and become depressed. One very wealthy retired CEO of a Fortune 100 company told Schlossberg, "Retirement is hollow."

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Luckily, for those interested in prolonging their working lives, opportunities should grow. With the number of younger workers stagnating, any expansion in the economy should force companies to tap older workers to make up shortfalls.

"Between 2005 and 2010, companies will probably be scrambling for older workers," says Ascher.

"For the past 25 years companies only wanted to talk to us about how to market to older Americans," says Bob Morison. "Now they're starting to ask how to employ them."