How would you plan retirement if you lived to 100?

A friend of ours recently had her 97-year-old aunt move in with her and her husband. She was still vital and healthy, but she'd run out of money. This got me thinking: How would you plan for retirement if you thought you'd live to 100?

Don't dismiss this exercise as a waste of time by thinking you won't live that long. Admittedly, the odds are against you living to age 100. But there's a good chance that you and your spouse or partner could make it to age 90 or 95. For example, the life expectancy calculator at the website of the Society of Actuaries shows that a 65-year-old man has a 17 percent chance of living to age 95 and a 33 percent chance (one in three) of living to age 90. A 65-year-old woman has a 23 percent chance (almost one in four) of living to age 95 and a 44 percent chance (almost one in two) of living to age 90.

All of the above odds assume your life expectancy reflects the characteristics of the general population. The odds of surviving are significantly higher if you live a healthy lifestyle regarding nutrition and exercise, don't smoke, don't abuse alcohol, and have above average income and/or above average education.

If you're currently in your 60s and thinking about retirement, it's a worthwhile exercise to prepare for the possibility that you or a loved one might live for a very long time. Even if you don't live to 100, you'll gain some important planning insights.

If your remaining lifetime could be 35 years or more, be prepared for lots of change. Think about the technological and political upheavals during the past 35 years. We experienced:

• High inflation in the early 1980s, with mortgage interest rates in the high teens
• Four stock market crashes
• The fall of the Berlin Wall
• Emergence of terrorism and climate change as major concerns
• Two wars involving American troops on foreign soil
• The impact of digital innovations such as the personal computer, the Internet, online shopping and social media
• Cellphones
• Skyrocketing health care costs
• Shifting responsibility for people's health and prosperity from the employer to individuals
• The emergence of 401(k) plans and the mutual fund industry

Need I say more? And you should expect more -- not fewer -- innovations and upheavals in the next 35 years.

One good friend of mine in his 80s reads incessantly, keeping up on all the latest technology developments and political events. He's connected with friends and relatives through email, social media and Skype. He's a good example to follow.

You should also maintain and nurture significant relationships with younger relatives and friends. They'll help keep you up-to-date, can pitch in to help if you have a crisis and will provide renewal of your social circle (a polite way of saying that you'll outlive many of your current friends and relatives).

You'll also want to hire doctors, accountants and financial advisors who are decades younger than you. Remember George Burns' reply when he was asked what his doctor thinks of his cigar smoking? "My doctor is dead." By choosing someone much younger, you can hope this person will be with you for all of your older years.

You'll also need to think about where you live. Will your current house and community support your lifestyle for the next 35 years? Many people want to downsize, be socially connected, become less reliant on cars and stay independent. Now's a good time to consider moving, before it becomes too hard for you to find a new place to live and uproot your current life. (Also learn about the "aging in place" movement, which has the goal of enabling you to live at home until the very end.)

What will you do for the rest of your life? Thirty-five years is a long time for recreation and play, which is the common depiction of retirement by the media. While grandchildren might be a fulfilling focus in the first decade or two of retirement, there's a good chance you'll still be alive when they move out and away and become young adults. You'll want to nurture interests, careers and causes that keep you connected and relevant to society.

Finally, it will take a boatload of money if you're truly retired and living just on the financial resources you've set aside, as my friend's aunt found out.

If you're in your mid-60s and end up living to age 100, you still have one-third of your life ahead of you. Don't think of retirement as winding down your life, starting on a path of decline and irrelevance. It's really a time of renewal and rejuvenation. So what will you do for the next 35 years?