I found a brief explanation online at wonderquest.com. Eye color is determined by the amount and distribution of melanin in the iris (melanin also gives our skin and hair its color). Apparently with age, the density (amount) of melanin can change in the iris resulting in the observed change in eye color. Similarly this can cause our hair to change color as we age. However, the perceived color of our iris does depend to a degree on having a dark area behind it and as we age the lens does yellow. That would make the area behind the iris less dark and therefore change the appearance of the iris. I would suspect that the changes in pigment density are far more important than changes in the lens, however. There are also other causes for changes in eye color that are mentioned in the linked column.