Bad habits, like smoking, become less appealing but more difficult to shake; more than 80 percent of adults who ever smoked began doing so before age 18. Still, men are smoking less than they did and the incidence of lung cancer in men is falling. Although the incidence of smoking -- 28.8 percent for black men, 27.1 percent for white men -- is similar, black men are at much higher risk of lung cancer than white men.

H.I.V. infection, the leading cause of death among men between ages 25 and 44, is often contracted during adolescence, when boys are experimenting with sex and are oblivious to the risks of infection. But with advances in drug therapies, the incidence of H.I.V.-related deaths has declined over the last four years.

MIDDLE AGE

Beginning in their early 40's, men experience a decline in testosterone of 1 percent each year. These reductions coincide with increased depressive symptoms, including anxiety and sexual dissatisfaction. While some doctors consider this stage tantamount to male menopause, others argue that the hormonal changes are too subtle to account for these symptoms. They note, too, that impotence and other conditions associated with middle age can be caused by ailments that tend to strike men in this age group, like diabetes.

The risk of heart disease, hypertension and diabetes is exacerbated by obesity, and middle age is when men are likely to be overweight. They lose 3 percent to 5 percent of their muscle mass for every decade after age 25. Reduced muscle mass and physical activity conspire to decrease men's resting metabolic rate. As men age, then, they burn less energy while resting and can gain weight even without changing their eating habits. And they do gain -- 2 to 3 pounds for every year over age 30.

Heart disease continues to be the leading cause of death for men in the United States. But the rate of heart disease-related deaths among men has decreased more than 50 percent since 1950; those who die of heart disease are dying later in life.

EARLY OLD AGE

Because men continue to produce testosterone throughout life, they are protected from -- though not immune to -- conditions like Alzheimer's Disease and osteoporosis. Their larger bone size also helps protect against this bone-weakening illness. Men can further maintain their mental acuity by engaging in intellectual activities. They can strengthen their bones and stem bone loss by undertaking weight-bearing exercise. The continued production of testosterone, however, can also adversely affect men. Testosterone aggravates hair loss and stimulates growth of the prostate gland. Noncancerous enlargement of the prostate occurs in more than half of men in their 60's and up to 90 percent of men in their 70's and 80's. At the same time, 80 percent of all prostate cancer cases occur in men age 65 and over.

LATER OLD AGE

Studies indicate that men are less likely than women to have difficulty maintaining normal routines, like bathing, dressing and using the toilet, as they age.

Still, the trend among the elderly in general is that they become less active, and so need fewer calories. Their appetites diminish, yet their nutritional needs increase because their bodies have lost the ability to synthesize and absorb important vitamins and nutrients. Their skin, for example, no longer easily synthesizes vitamin D when exposed to the sun. The benefits of avoiding potentially harmful foods, such as those high in cholesterol, lessen with age. Maintaining weight and making sure the right nutrients are present in the diet become more important.