The active kidney issue in made up from nephrons (see the picture), which filter blood and remove the waste from blood into a fluid that later becomes urine.
As we age, the number of nephrons slowly decreases. In addition, kidneys are dependent of the blood supply, just like other organs, except even more so. The vessels that bring blood to kidneys may harden with age, similarly to vessels in other parts of our bodies. As a consequence, blood supply may decrease, which may result in decreased kidney function. However, kidney function remains normal as long as we stay healthy. On the other hand, it can be affected by a number of illnesses that we may suffer from as we age.