From Scientific American:
Exercise has been shown to help the heart, whereas a lazy lifestyle can be a major risk factor for heart disease. But few studies have examined how exercise impacts health at different ages. Now researchers have shown in a small study that even those who take up exercise after age 40 derive significant health effects. Epidemiologist Dietrich Rothenbacher of the University of Heidelberg and his colleagues surveyed 312 patients–mostly men–between the ages of 40 and 68 who suffered from coronary heart disease and 479 volunteers matching the patients in age and sex. The scientists asked them to detail their physical activity from the ages of 20 to 39, 40 to 49 and 50 years and older. More than 10 percent of patients and 6 percent of the controls admitted to lifetimes devoid of physical activity.
Compared to these inactive counterparts, those who were active throughout their lives enjoyed more than a 60 percent less chance of developing heart disease. But even those who became active only after the age of 40 enjoyed a 55 percent less chance of cardiovascular trouble, and those who went from being inactive to very active saw the greatest benefits.