Lisa Gable isn't one to sit back as the world changes around her. At 84, Gable, who designed and sells the Strap-Mate bra strap holder, still travels to business seminars and takes the occasional night school class to keep on top of the accessories market. She's open to just about any new development, except one: Retirement.

"I think most of my life, I've wanted to be a useful person and to be of help to other people," says Gable, who founded her company at age 70. "And it's very gratifying to be able to continue to do that."

Older entrepreneurs like Gable -- along with the baby boomer generation not too far behind -- are rewriting the rules of retirement. Today, small-business owners and their employees are living longer, healthier, and more productive lives than ever before. The average male who drops out of the workforce at 65 can now expect to live another 16-18 years in retirement (even longer for women), compared to 13-15 years back in the 1980s, according to AARP. A decade ago, 30 percent of the total population of all the G7 countries combined was 50 or over. Within the next 10 years, that figure will grow to 40 percent. Over the same period, the population between ages 15 and 49 -- traditionally considered prime working years -- will fall from 51 percent to 44 percent.

Given these demographic shifts, it's not surprising that people around the world are starting to have second thoughts about retirement. U.S. labor force participation by 50- to 64-year-olds was 70 percent in 2005, up from 64 percent in 1970, labor statistics show.

In part, these gains are being driven by employers who are facing an increasingly tight labor market and looking to get more out of their mature workers. In recent years, more businesses have begun rehiring retirees, or offering part-time or part-year working schedules, flexible hours, and telecommuting, according to a recent AARP study. They're also implementing age diversity training programs, support services for senior parents and spouses, and retirement transition options -- all with the stated goal of hanging on to their aging workers.

Meanwhile, employers themselves aren't expected to bow out any time soon. In a recent national survey of 750 small-business owners, nearly half said they never intend to retire at all, according to the National Federation of Independent Business, a Washington-based advocacy group. Given that investments are a major source of retirement income for most owners, the group blames weaknesses in the stock market over the past five years for shelving many retirement plans.

Yet beyond finances, the NFIB says its findings "match the high-energy profile often put forward as characteristics of business owners." In other words, having been hard-driven, risk-taking entrepreneurs most of their lives, business owners are not likely to turn into beach bums at 65, 70, or 80.

"What would I do with my time? Why should I give up doing something I like to do?" asks Bernard Rothzeid, 82, a founding partner of the Manhattan-based architectural firm, Rothzeid Kaiserman Thompson and Bee.

Phyllis Apple, the 84-year-old CEO of the Apple Organization, a North Miami Beach, Fla.-based public-relations firm, agrees. "I can't see myself playing golf everyday. That would just be a bore. And I'm enjoying good health. I'm still very blessed."

Other older entrepreneurs see their late-life ventures as having an important social mission they simply can't abandon. Bob Galvin, 85, who spent three decades as CEO of Motorola, is spending his twilight years reinventing the nation's unreliable power grid and solving big-city gridlock.

For his part, Jack Weil, the 106 year-old CEO of Rockmount Ranch Wear, a family-run western apparel company based in Denver, still goes into work every day to spend time with his grandson -- that, and to chat up customers who visit the store from all over the world.

"It's exhilarating," Weil says. "It's what keeps you going."