Last weekend, singer-songwriter Leonard Cohen planned to celebrate his 80th birthday with a cigarette. He had announced last year that he would resume smoking when he turned 80. "It's the right age to recommence," he explained.

At any age, taking up smoking is not sensible. And yet, Cohen's plan presents a provocative question: When should we set aside a life lived for the future and instead, embrace the pleasures of the present?

At the start of the 20th century, only 0.5 per cent of the US population was over age 80. Industrialised nations were preoccupied with infectious diseases such as tuberculosis and polio. Many of the common diseases of ageing, such as osteoporosis, were not even thought of as diseases.

Today, 3.6 per cent of the population is older than 80, and life is heavily prescribed not only with the behaviours we should avoid, but also the medications we ought to take. More than half of adults aged 65 or older are taking five or more prescription medications, over-the-counter medications or dietary supplements, many of them designed not to treat suffering but to reduce the chances of future suffering. Stroke, heart attack, heart failure, kidney failure, hip fracture - the list is long, and with the Department of Health's plan to prevent Alzheimer's disease by 2025, it grows ever more ambitious.

Ageing in the 21st century is all about risk and its reduction. Insurers reward customers for regular attendance at a gym or punish them if they smoke. Doctors are warned by pharmaceutical companies that even after they have prescribed drugs to reduce their patients' risk of heart disease, a "residual risk" remains - and more drugs are often prescribed.

One fitness product tagline captures the zeitgeist: "Your health account is your wealth account! Long live living long!"

But when is it time to stop saving and spend some of our principal? If you thought you were going to die soon, you just might light up, as well as stop taking your daily aspirin, statin and blood pressure pill. You would spend more time and money on present pleasures, like a dinner out with friends, than on future anxieties.

When it comes to prevention, there can be too much of a good thing. Groups such as the US Preventive Services Task Force regularly review the evidence that supports prevention guidelines and find that after certain ages, the benefits of prevention are not worth the risks and hassles of testing, surgery and medications.

Recent guidelines for cholesterol treatment from the American College of Cardiology and the American Heart Association, for example, set 79 years as the upper limit for calculating the 10-year risk of developing or dying from heart attack, stroke or heart disease. They also suggest that, after 75, it may not be beneficial for a person without heart disease to start taking statins. But that doesn't mean everyone follows this advice.

Besides, isn't 75 the new 65? Age seems a blunt criterion to decide when to stop. Is Cohen at 80 really 80? In his mid-70s, he maintained a rigorous touring schedule, often skipping off the stage. Maybe 80 is too young for him to start smoking again.

Advances in the science of forecasting are held out as the answers to these questions. Researchers at the University of California, San Francisco, and at Harvard have developed ePrognosis, a website that collates 19 risk calculators that older adults can use to calculate their likelihood of dying in the next six months to 10 years. The developers of ePrognosis report that frail, older adults want to know their life expectancy so they can not only plan their health care but also make financial choices, such as giving away some of their savings.

Even more revolutionary is RealAge, a product of Sharecare Inc which has quantified our impression that as we age, some of us are really older, while others are younger than the count of their years. It uses an algorithm that assesses a variety of habits and medical data to calculate how old you "really" are.

Websites such as these can be a convenient vehicle to disseminate information (and marketing material) to patients. But complex actuarial data is best conveyed in a face-to-face conversation with a doctor.

We are becoming a nation of planners living quantified lives. But life accumulates competing risks. By preventing heart disease and cancer, we live longer and so increase our risk of suffering cognitive losses so disabling that our caregivers then have to decide not just how, but also how long, we will live. Bioethicist Dena Davis says that emerging biomarkers that may some day predict whether one is developing the earliest pathology of Alzheimer's disease (like brain amyloid, measured with a PET scan) are an opportunity for people to schedule their suicide. Or at least start smoking.

Our culture of ageing is one of extremes. You are either healthy and executing vigorous efforts to build your health account, or you are dying. And yet, as we start to "ache in the places where (we) used to play", as one of Cohen's songs puts it, we want to focus on the present.

Many of my older patients and their caregivers complain that they spend their days going from one doctor to the next, and data from the National Health Interview Survey suggests one reason: Among older adults whose nine-year mortality risk is 75 per cent or greater, as many as half are still receiving cancer-screening tests that are no longer recommended.

I don't plan to celebrate my 80th birthday with a cigarette or a colonoscopy, and I don't want my ageing experience reduced to an online actuarial accounting exercise.

I recently gave a talk about Alzheimer's disease to a community group. During the question-and-answer session, one man exclaimed: "Why doesn't Medicare pay us all to have dinner and two glasses of wine once a week with friends?" What he was getting at is that we desire not simply to pursue life but also happiness, and that medicine is important, but it's not the only means to this happiness. A national investment in communities and services that improve the quality of our ageing lives might help us to achieve this.

The writer is a professor of medicine, medical ethics and health policy at the University of Pennsylvania.

NEW YORK TIMES