When you reach the age of retirement, you also reach what we might call the “age of aging.”

Of course, we age from the moment we are conceived. But during childhood and on into our twenties, aging is mostly a process in which our capabilities are expanding. Even in our thirties and forties, while we begin to show and feel modest declines in our physical selves, our experience provides skills and knowledge that compensate – often more than compensate – so that these can be our years of peak overall capability. But the declines become more pronounced in our fifties and sixties. After that, we are likely to be considered by others to be elderly, and may well feel that way ourselves.

A generation or two ago, retirement was a phenomenon of the elderly. “Retired” usually meant over the hill – too old to work, too disabled, or both. Today, retirement is more often considered a phenomenon of late middle age. Working people expect to retire while they are still young and healthy enough to enjoy it.

Given that nowadays someone retiring in his or her early sixties is likely to live about another 25 years, most of which will probably be healthy years, this more ambitious view of “retirement” is entirely justified.

Yet we will still be aging in those years, and declining at a more rapid clip than we were before. Whatever we choose to do, we will do with somewhat more difficulty. We will have less physical strength, less dexterity, diminished stamina, a little less mental sharpness, a harder time remembering new things. These will not disable most of us, not in our sixties or even our seventies, but we will feel them. And the longer we hang in there, the more powerfully we will feel them.

We will also be more susceptible to illness. We will gradually become more accident-prone. We will heal and recover more slowly. Eventually – and for many of us much sooner than we expect – we will develop some condition that slows us down a lot, either physically or mentally. And at some point, of course, we will die.

Does any of this sound depressing? If so, then coming to terms with it is one of your major tasks in retirement.

All the happy talk about retirement can be true. It can be a time of newfound freedom and fulfillment. You might have years and years, maybe decades, for fruitful efforts in whatever direction your fancy takes you, for enjoyment of life, for time with your partner and children and grandchildren and other family and friends. Making the most of this opportunity is a worthy goal for your retirement years.

But each stage of life, while having its own aims and purposes, also should be a preparation for the next stage. So while you are retired, and doing your best to make it what you want it to be, you should also be paying attention to the signs of aging. They are important reminders that the party does not go on forever, and that you should be getting ready for what could be next: losses not only in terms of your own mental and physical abilities, but corresponding losses in others who are close to you, and if you are lucky enough to live into true old age, loss of most of your age-mates.

What does such preparation mean?

First, it means not kidding yourself about being on the downward slide. However gradual it may turn out to be – and it may not be gradual at all – you are already too old to die young, and you will either decline slowly and die, or you will decline more quickly and die. Whether this scares you are not, the earlier you accept the idea, the happier your retirement years will be.

Second, after learning to keep awareness of aging and death at least in the back of your mind, you need to learn to get comfortable with it. Here is where your philosophy of life – including your religion, if you have one – gets put to the test. Whatever you say you believe about the immortality of the soul, about the reincarnation of the spirit, about the peaceful oblivion of death, or whatever other view you subscribe to, this is the test of it. If you are not at peace with the idea of aging and the idea of dying some day, then your beliefs are not working as well for you as they should, and you would be doing yourself a favor to examine them more closely. Maybe you need new beliefs, or maybe you just need to better understand (and maybe recommit yourself to) the ones you already have.

Third, you should think about the practical consequences of aging and dying. And you should not put this off. However healthy you are now, you could be stricken by a debilitating disease at any time, or even die suddenly. Undoubtedly you know people, probably quite a few of them, to whom these things have happened. We all think, “Well, it won’t happen to me.” Accepting and understanding aging means that you start to think differently. It most certainly will happen to you, you just don’t know how or when. But the older you get, the more likely that it will be soon. So it is important to be ready psychologically, and also in other ways: keeping your affairs and records in order, making sure you have appropriate legal and financial arrangements and that at least two or three people close to you know what these arrangements are and where to find your papers.

Fourth, it means considering your legacy, and perhaps adjusting your current life accordingly. What will your family and friends remember of you when you are gone (or alive but too disabled to still be “in the game”)? What will people have learned from you? What positive effect will you have had on your community? Will people sense a loss when you are gone? Will they feel that they were better off for your having lived? If you are not sure about the answers, or you are well aware that your legacy is not going to be what you would like it to be, then fortunately you probably still have time to make corrections.

I’d like to come back to all of these issues in later blogs, but for now, the message I want to leave is that retirement is not just about living the good life as long as you can. It is also about learning to deal with what’s coming, so that when it does come, you handle it with equanimity and grace, and with the minimum of regret.

Chuck Yanikoski is a retirement adviser who lives and works in Harvard. For more about him, visit http://www.ChuckYRetirement.com.