BALTIMORE — Here's more proof that exercise is beneficial at any age. A 20-year study funded by the National Institute on Aging found that while the capacity for aerobic exercise may decline as a person ages, an active lifestyle can slow the process. The amount of oxygen the body consumes during exercise declines 3 percent to 6 percent per decade in people in their 20s and 30s, but falls more than 20 percent in those in their 70s and beyond, according to the Baltimore Longitudinal Study on Aging. But by participating in a training program, individuals can raise their aerobic capacity 15 to 25 percent.