Stay ahead of the game   Midlife exercise brings greater fitness in years to come The level of fitness people have declines with every decade of life. Aerobic capacity translates into fitness. It's the amount of oxygen consumed while exercising. Researchers for the Heart, Lung, and Blood Institute have found that exercising to increase aerobic capacity has its benefits throughout life. Though a decline in capacity is inevitable with each passing decade, aerobic exercise counteracts part of the decline. In the study, subjects in their 20s and 30s had an aerobic decline of 3 to 6 percent per decade. Those in their 70s experienced a 20 percent decline per decade. Study subjects were men and women ages 21 to 87. They were tested every four years. All of that decline is not unavoidable, however. People who raise their capacity by 5 percent in a decade where it normally decreases 10 percent, for example, would then have only a five percent decrease.  If they increase aerobic capacity by 5 percent in the next decade, they really have made a significant stride toward fitness in later life. Exercising can't hold off the effects of aging, but it can reduce them. It can increase an elderly person's chances of having an independent lifestyle.