MANY PEOPLE, starting in their 50s, feel challenged by not just the question of whether or when to retire, but with the very concept of retirement.

Starting in the 20th century, Americans began to have the luxury of retirement. They could work for one company for a very long time and then stop work completely. For the majority of that century, many Americans could fall back on employer and government retirement income and medical coverage, but the average retiree lived only a few years after retiring at 65.

That's changing fast. The average retiree lives for more than 20 years after retirement because many are retiring earlier and living much longer. While some people don't choose to retire until they're in their 70s, many are completely retiring, retiring in stages, or cycling between full-time, part-time and no employment in whatever order they choose. Leisure is becoming more of a choice than a mandatory occurrence at a certain age.

Ken Dychtwald, internationally recognized aging specialist and founder of the Age Wave, a Bay Area company that consults with corporations on aging matters, found in a survey that baby boomers either want to alternate between work and leisure (48 percent); never work for pay again (17 percent); work part time (16 percent); start a business (13 percent); or work full time (6 percent).

A desire to keep mentally and physically active and connect with others drives the 83 percent who want to work even more than having health benefits or more money, according to the survey.

If you are in the second half of your life, you may be asking yourself not so much whether you want to work but how you want to balance work and leisure. Work for you may mean some combination of paid employment, volunteering and hobbies. Leisure may mean travel and learning as well as time with family and friends and entertainment.

This picture can sound rosy but, like all things in life, we're faced with mixed blessings. As we live longer, more of us face chronic diseases like Alzheimer's and Parkinson's. By the middle of this century, 14 million Americans are expected to suffer from Alzheimer's and tens of millions of older adults, including debt-ridden boomers, may live in poverty.

Right now, 40 million retirees spend an average of 43 hours a week just watching television.

Medical breakthroughs, resolution of the impending Social Security and Medicare crises and other impossible to predict changes can improve the outlook for a vibrant, secure and choice-filled second half of life.

But in these changing, challenging times, each of us needs to discern what we want, what is realistic, and how to achieve our work and leisure dreams as individuals and people in a community.

If you discover and pursue what will enliven you in your mature decades, you will need support from people who are close to you and from kindred spirits. Don't just envision your future. Share it and commit yourself to working for change to make your and others' retirement dreams come true. Here are some resources to help you envision and make connections:

-"Age Power: How the 21st Century Will Be Ruled by the New Old," by Ken Dychtwald (Tarcher/Putnam, $13.95).

- "The Longevity Revolution: As Boomers Become Elders," by Theodore Roszak (Berkeley Hills Books, $15).

- "Too Young To Retire: 101 Ways To Start the Rest of Your Life," by Marika and Howard Stone (Plume, $13).

- Visit http://www.2young2retire.com or http://www.retirementliving.com.

Sandra J. Cohen, R.N., and Roger Cormier are consultants who help East Bay families plan and coordinate care of an older relative at home or in a care facility. You can reach them at (510) 652-3377 or (925) 945-8855 or visit http://www.eldercaremanagers.com.