The U.S. population rose 13 percent between 1990 and 2000, the Census Bureau says, but the over-85 population rose 38 percent, to 4.2 million. Another way to look at this is that in 2000, an estimated 2 percent of the population was age 85 and older. By 2050, the percentage in this age group is projected to increase to almost 5 percent of the U.S. population.

Blood vessels also lose some elasticity, says geriatrician Thomas A. Cavalieri, D.O., professor and chairman of medicine at the University of Medicine and Dentistry of New Jersey-School of Osteopathic Medicine, in Stratford, N.J.

It's important to keep exercising throughout your life to help preserve these functions. Exercise should cover all four of these areas: endurance (to help your breathing and heart rate), strength (to build muscles), balance (to help prevent falls), and flexibility (to keep your body limber).

"Routine weight-bearing exercise can also help decrease the rate at which bone is lost through aging," says Dr. Cavalieri. Ask your doctor before beginning any new exercise program.

Don't forget to drink plenty of liquids throughout the day, says the National Institute on Aging (NIA). In addition to water, other good choices are juice, milk, and soup.

People with close friends survive heart attacks longer than those who are isolated, says Dr. Cavalieri. "Social isolation is associated with depression and poor health outcomes."

One shot usually offers years of protection against a serious health threat that rises with age.

No matter what your age, as soon as you stop smoking, your heart and circulatory system begin to improve, the NIA says. Your risk for heart attack and stroke begins to drop. Although quitting smoking can't reverse permanent damage to the lungs, it may help slow additional damage.

Bone mass reaches maximum density between ages 25 and 35, then begins to decrease, as the bones lose calcium and protein. As women enter menopause, they can lose up to 3 percent of their bone mass each year, the NIA says. This usually continues for at least a decade, with up to half the skeletal density lost. Both men and women have an age-related decrease in bone of approximately 1 percent a year after age 50. "Exercise, calcium and vitamin D intake, along with some medications, have been shown to increase bone mineral density," says Dr. Cavalieri.