LONDON — So now it is official: like her late friend and comrade in arms Ronald Reagan, Margaret Thatcher is suffering from Alzheimer’s. According to her daughter Carol, dementia was first diagnosed eight years ago and the former prime minister’s condition has been deteriorating gradually ever since.

It was generally known that Lady Thatcher, 82, had suffered a number of minor strokes, but until now her memory loss — which sometimes makes it hard for her to finish sentences — had been hidden from the public.

From my own brief encounters with Lady Thatcher during the past 25 years, I can testify that she is still capable of occasional flashes of the old fighter. But for at least a decade she has been fighting a losing battle with Alzheimer’s that is sometimes painful to behold. In a newly published memoir, “A Swim-On Part in the Goldfish Bowl: A Memoir,” her daughter describes how Lady Thatcher kept forgetting her husband Denis’s death in 2003, forcing her to relive the bereavement every time she was reminded.

The problem epitomized by the woman many consider Britain’s greatest peacetime prime minister is of course a consequence of a rapidly ageing population. In Britain, the growing numbers of the elderly have recently exceeded the number of children for the first time, and the balance is set to tilt still further towards age and against youth. This is of course the dominant demographic phenomenon almost everywhere in Europe, and unless the baby boom generation addresses the problem soon, before it is too late, their addition to the greying population will sooner or later create a colossal health care, welfare, and pension crisis.

However, a less frequently noted fact is that the rising incidence of Alzheimer’s and related problems of old age are really symptoms of a physical constitution robust enough to have survived far beyond what was previously considered the natural human span. By the time my father had reached 50, my present age, all but one of my grandparents had died. My children are fortunate enough to have all four grandparents not only alive, but highly active and even productive.

One theory about the unprecedented longevity and health of the generation that lived through World War II is that, in Britain at least, they were subject to food rationing. This is practically the last fact that Alzheimer’s sufferers forget. It is characteristic that Lady Thatcher, the daughter of a grocer, can still talk animatedly about how to make the most of the unappetising food, such as powdered egg, that she ate as a young woman during and after the war. Rationing lasted for more than a decade, between 1939 and 1954, until Winston Churchill returned to power and scrapped the remaining wartime controls.

It is often asserted that the British have never eaten more healthily than in the 1940s, the decade of rationing, and it may be true. Though the science of nutrition was still in its infancy, there was a growing awareness of the importance of fresh vegetables and fruit. The U-boat blockade in the North Atlantic seriously attempted to starve the British in both world wars, but never came close. Still, under such circumstances obesity was never going to be a problem, and the survivors of the wartime generation seem to have laid the foundations for a robust old age.

I know numerous octogenarians, and even a few nonagenarians, who are still working. When the Russian writer, Alexander Solzhenitsyn, died earlier this month, I approached the historian and poet Robert Conquest to write a cover story for Standpoint, the monthly magazine of which I am the editor. Mr. Conquest, who knew Solzhenitsyn and translated his long narrative poem Prussian Nights, is actually the Russian’s senior by two years. (Solzhenitsyn was 89, he is 91.) He had to write the article in less than a week and update it to take account of the Russian invasion of Georgia. Bob Conquest met the deadline, seemingly effortlessly.

I am sure that such mental activity is at least as important as diet. The game that I play most, chess, has been shown to be an effective prophylactic in warding off Alzheimer’s, and I am sure that there are many other forms of exercise for the brain that have the same effect. On average, the more highly educated you are, the slower the onset of Alzheimer’s.

Then old-fashioned word for Alzheimer’s, “senile,” really means “extremely old.” It was evidently once taken for granted that most of the few people who did live to a great age would be “in their dotage” — could, in other words, be expected to have “lost their minds.” That is no longer true, if it ever was, though over the age of 80 about one person in five suffers to a varying degree from the loss of memory and concentration. Scientists may soon be able to slow or even halt the progress of Alzheimer’s and other forms of dementia.

For the present, however, these degenerative diseases of the brain remain a source of great sadness and sufferers deserve all our compassion. Let us salute Margaret Thatcher, once nicknamed the Iron Lady by the Soviets, whose own daughter thought of her as “ageless, timeless and 100 per cent cast-iron damage-proof.” She is, of course, mere flesh and blood and, looked after by a loyal staff and devoted daughter, shares her human frailty with the rest of us. I hope that she can continue to enjoy her declining years, and that when the time comes she will receive the kind of fond farewell that millions of Americans gave to Ronald Reagan. Let us now praise famous men — and women too.

Mr. Johnson is the editor of Standpoint.