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Cause
Each hair follicle contains pigment cells which incorporate melanin, in varying colors, into hairs as they grow. When cats age, these pigment cells can die in a similar way to humans. This results in a complete lack of pigment in a hair causing it to appear bright and white. Gray looking hair is the overall appearance of a mixture of pigmented and white hairs.
Trends
It is not possible to predict when or if a cat will get gray hair. Like humans, a cat may get gray hairs at a relatively early age or may not get any at all. Generally, graying is not particularly common in cats and most of them will remain a consistent color for their whole lives. Whether a cat gets gray hair or not, veterinarians at Petplace.com describe how at around ten years old a cat often starts acting like a "senior" cat and becomes less active.
Breed Tendencies
Some breeds of cats vary in their tendency for pigment loss. For example, Siamese cats are vulnerable to two conditions that result in a "graying" appearance as described by veterinarian Mike Richards on Vetinfo.com. The first is vitiligo, which is quite rare and has only been seen in female cats, as reported by Muller and Kirk's Small Animal Dermatology, 6th edition by Scott, Miller and Griffin. Vitiligo results in a loss of pigment to the feet and facial areas and has no known negative affects on health.
Other Conditions
Periocular leukotrichia is more common than vitiligo. The fur around the eyes lightens so that the cat appears to be wearing goggles. This condition has been linked with infections of the upper respiratory system and also Horner's Syndrome.
Gray or white patches are also associated with scar tissue or trauma. Cats who have been involved in many cat-fights or had injuries may show white or grey patches that grow around their old wounds.