At the weekend what a number of us have known for a long while became public: Margaret Thatcher is suffering from dementia. Since 2000, and aggravated since by a series of small strokes, the Iron Lady has found her extraordinary mind slipping away from her.

As her daughter Carol describes it in a new memoir, the first time Lady Thatcher’s memory lapse became obvious, “I almost fell off my chair. Watching her struggle with her words and her memory, I couldn’t believe it. She was in her 75th year but I had always thought of her as ageless, timeless and 100% cast-iron damage-proof. The contrast was all the more striking because, until that point, she’d always had a memory like a website.”

Lady Thatcher’s condition brings to mind the demise of her great personal and political friend, Ronald Reagan, whose final years were devastated by Alzheimer’s. Nancy Reagan uncomplainingly fulfilled the heartbreaking task of nursing him as his mind drifted into oblivion.

Lady Thatcher has not quite reached that stage. She has a memory, but it mostly recalls times long past, including happy moments when her husband of 50 years, Denis, was alive. Or she recalls serving groceries from behind the counter of her doting father Albert Roberts’s grocery store. It is particularly cruel that she cannot call to mind her many triumphs, cannot remember those who stayed loyal to her after her ignominious removal from power, and cannot always recognize her friends and family.

There is something particularly moving about the plight of someone once so inspiring and powerful reduced in what Shakespeare dubbed “the seventh age of man” to a “second childishness and mere oblivion, sans teeth, sans eyes, sans taste, sans everything.” It is not just the bathos of a powerhouse left helpless and vulnerable, but that those who changed the world for the better should be deprived of the decorum in old age that is their due.

You would not know it from talking to most Brits, but the reforms Lady Thatcher urged upon Britain’s sluggish economy and on its political, social, and business culture, and her resolution of the national quandary so aptly observed by Dean Acheson — “Great Britain has lost an empire and has not yet found a role” — have left them prosperous as never before. London is now the undisputed, if unofficial, capital of Europe, a magnet for every young European who wants to get ahead.

Her legacy is not just in unearthing the entrepreneurial zeal that had been heaped beneath years of benign neglect by well meaning souls who set out to shield the British from the vicissitudes of the free market, but in the sense of national purpose she restored to Britain and the free world by the simple act of standing up to bullies.

It was after her triumph over the strutting Argentine general, Leopoldo Galtieri, in the Falklands — a victory that transformed her fortunes from a likely one-term failure into a worthy successor of Boadicea and Elizabeth I — that she emerged as a powerful presence on the world stage. From her new vantage point she went on to battle a succession of tinpot tyrants, including the communist gerontocracy clinging to power in the Kremlin.

Since her party ousted her in 1990 in a typically passionless British coup, she has remained a prophet largely ignored in her own country. While Americans worship her, the British have found it hard to forgive the hectoring and sometimes brutal fashion in which she harried them into changing their ways. It is of little credit to Tony Blair, who shares Lady Thatcher’s fate of being an American idol derided at home, that he vetoed the state funeral she deserves. (To his credit, Gordon Brown swiftly overturned Mr. Blair’s mean verdict.)

Like Mrs. Reagan, Carol Thatcher has, in describing her mother’s condition, raised a topic we mostly prefer to ignore. “Sufferers look and act the same, but beneath the familiar exterior something quite different is going on. They’re in another world and you cannot enter,” she writes. And, typical of the daughter who shares many of the compassionate virtues of her maternal grandmother, Beatrice Stephenson, Carol fully realizes that in many ways still Lady Thatcher is fortunate.

“I had to remind myself,” she writes, “that we were lucky. Friends had to drop everything to race to opposite ends of the country to attend to ailing parents who lived alone or without proper supervision. My mother would always have a driver and a 14-hour police guard, not to mention enough funds to afford the appropriate nursing care.” It is a point worth making: many of us — perhaps most of us — can expect to end our lives in loneliness, discomfort, penury, and distress.

In 10 weeks we shall decide whether John McCain, who will be 72 on Friday, is too old to be president. Yet, while age is a key factor in the election, it is not an issue. Worse, the senator’s age is the butt of snide jokes by late night comics. To talk of gender, race, height, rotundity, and sexuality is off limits; poking fun at old people is, apparently, fair game.

We live in a world where more of us are living longer, presenting new challenges to health care, accommodation, working practices, pensions, above all demanding a more generous and understanding attitude toward the aged. If even Lady Thatcher and Reagan were ill prepared for what fate cruelly dealt them in later life, what chance that the rest of us will be ready? In brief, it is time we all grew up.

Mr. Wapshott is the author of “Ronald Reagan and Margaret Thatcher: A Political Marriage,” published by Sentinel.