A study observes that subjects who say they exercise regularly reported only half as many serious illnesses per year, on average, as those who say...

A study observes that subjects who say they exercise regularly reported only half as many serious illnesses per year, on average, as those who say they do not exercise regularly. The results section in the article states, "We next analyzed whether age was a confounding variable affecting this association." Explain how it is that age could potentially explain the association between exercising and illness.