If you’re an 80-year-old, it may make you ner­vous to hear that life expectancy is about 82 years for American women and about 76 for men. But that’s life expectancy at birth. At age 80, American women can expect to live, on average, another 9.7 years, men another 8.4 years; at age 90, women live 4.8 more years, men 4.1 years, according to conservative esti­mates by the Social Security Administration. Even at 100, average life expectancy is another 2.5 years.

Those are population-wide averages. Obvi­ously, your health affects your life expectancy. Having heart disease, diabetes, or another seri­ous condition increases the chance that you will die prematurely, as does smoking and being very overweight or very underweight. (So does being poor, though that is rarely considered in health-related life expectancy calculations.)

Study after study has shown that people are not good at estimating how long they are likely to live based on their age and health—or don’t even want to discuss it. The latest study on how well people judge their life expectancy was conducted at the University of California, San Francisco, and published in JAMA Internal Medicine. It involved 2,018 Americans (ages 64 to 89 initially), who were asked to assess their chances of living to various ages. Their guesses were compared to an objective prog­nostic calculator as well as the actual survival rates of people their age over the next ten years.

Overall, a little more than half of the partici­pants were relatively accurate in their estimates, coming within 25 percent of the objective cal­culation; one-third underestimated by more than that; and one-eighth overestimated by more than that. The older they were, they worse they were at predicting. Those in their eighties or nineties were more likely to be too optimis­tic in their guesses, while those in their sixties or seventies were far more likely to underesti­mate their future years.

Does it matter, though, if you estimate your life expectancy accurately or not? Yes, according to the UCSF researchers. An accurate estimate can help in making certain medical decisions. For instance, various guidelines say that people with a predicted life expectancy of less than five or ten years should stop being screened for cancer, since it’s likely they won’t live long enough to reap the benefits but may experience the short-term harms. Those with a shorter life expectancy who overestimate their survival may opt for ill-advised testing and aggressive treatments. On the other hand, people who mistakenly think they won’t live long enough “may choose to forgo interventions that are likely to help them.”

No one has a crystal ball, of course, and there will always be people who buck the odds. But if you are older and facing major medical decisions, you’d be wise to broach this subject with your doctor

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