Many people lose their ability to clearly distinguish certain colors as they age, with losses typically starting around age 70 and getting worse over time. The loss of visual acuity as you age is experienced as an actual physical impairment and should be treated accordingly.
The reason why the vision of color is affected is not 100% clear. The photoreceptor cells in the Retina, called Rods and Cones, which are responsible for normal color vision, the decline in sensitivity as we age, causing colors to become less bright and the contrast between different colors to be less noticeable.