The population of people who are 65 years old or older will double within the next 25 years, according to a report from the U.S. Centers for Disease Control. How normal aging progresses depends on both genes and lifestyle choices. Changes in skin, hair and hearing are expected. But some people don't experience other physical and mental changes related to aging — like alterations in mental processes, the function of the immune system, vision and bones — and reasons are difficult to identify. What we do know is that keeping mentally and physically active as they age can help nearly everyone maintain good health.

Take this quiz to find out how much you know about normal aging.