People who quit smoking, even at the ripe age of 75, could be boosting their life expectancy, according to new research.

The findings suggest it is never too late to make healthy lifestyle choices.

Public health scientists at the University of Michigan (U-M) say their research supports the "profoundly important assessment that quitting smoking is the single best thing people can do to enhance their life expectancy."

That knowledge could seriously motivate people who have been smoking their whole lives, and who may have resigned themselves to a long-term elevated risk of cancer or cardiovascular disease.

One study of nearly 13,000 middle-aged men found those smoking 30 or more cigarettes a day at the start of the investigation faced a 21 percent greater risk of dying over the following 25 years.

But these risks are not set in stone. Numerous studies have found that quitting smoking at any age is followed by short-term health benefits, and researchers at U-M have now found the long-term benefits are not just limited to young and middle-aged adults who quit.

Their estimates suggest that people between the ages of 35 and 75, who used to smoke in the past, have added considerable time back to their life expectancy by giving up cigarettes.

Using publicly available, nationally representative health data, the team performed a series of calculations to estimate the average number of years lost due to smoking and the average years gained by quitting smoking at various ages.

The quitting ages considered were 35, 45, 55, 65, and 75.

The life expectancy of those who quit was compared to those who kept smoking at these specific ages and those who had never smoked at these ages, and these estimates were graphed based on probability.

Compared to those who never smoked, individuals who smoked at age 35 and continued smoking throughout the remainder of their lives were predicted to lose about 9 years of life on average (23 percent of their total life expectancy).

By contrast, people who stopped smoking at age 35 only had about a year difference between their life expectancy and that of those who never smoked. In other words, they had regained, on average, about 8 years of expected life.

The findings are supported by a previous study from researchers in Canada and Norway, which found that if a person can stop smoking for a decade, they can potentially avert a decade of life lost. But even at older ages there were benefits to life expectancy.

The current research agrees. Individuals who smoked at age 75 and who continued for the rest of their lives were predicted to lose about 4.4 years of life on average (nearly 50 percent of their life expectancy at this age).

Based on probability curves, however, the chances of gaining at least 1 year of life among those who quit smoking at age 65 and 75 are 23 percent and 14 percent, respectively. And that's nothing to scoff at.

Clearly, the most benefits come when smokers quit sooner rather than later, but that doesn't mean quitting at an older age is a waste of time. It could actually gain you time.

"These findings may be valuable for clinicians seeking scientific evidence to motivate their patients who smoke to quit," conclude researchers at U-M.

The study was published in the American Journal of Preventive Medicine.