I can think of four ways that domestic cat fur changes with age, two of which are caused indirectly. I will start with one example. There is an overlap between cats and humans on how ageing affects hair.
Matting in older cats due to neglected self-grooming (autogrooming)
We know that old cats self-groom less well and/or less often because (1) motivation drops off due to tiredness and (2) they are less flexible. Normally cats are amazingly flexible which allows them to groom 90% of their body but old age makes certain areas out of bounds.
Longhaired cats such as the Maine Coon and Persian can suffer from matted fur unless their caregiver assists by regular grooming. This change in fur quality is an indirect consequence of old age by which I mean this is not a change in the anatomy in older cats.
Greying hairs around the muzzle
I have seen this more often in dogs, but it happens in cats too. The hairs strands become grey in parts of the coat as human hair does for a lot of people. The cells in the skin that produce the pigment that is deposited in the hair strands (melanocytes), and which gives the coat its colour and pattern, stop functioning. They no longer produce the pigment melanin. Without melanin the appearance of each fresh hair strand after shedding is more transparent, white or grey. This is a direct product of ageing.
There are reports that hyperthyroidism can cause hair greying because the disease affects the cells of the body and apparently the melanocytes. Hyperthyroidism is associated with elderly domestic cats. This is another indirect cause of greying in old age.
Hypo and hyperthyroidism can also cause hair loss which is a clear change in the coat linked to age which leads me to the next section.
Thinning hair in some cats
As is the case with humans, old age can lead to thinning hair. My research indicates that one reason for this is because reduced self-grooming causes increased hair shedding. However, I am not sure of the logic in that statement. It implies that self-grooming which removes loose hair strands encourages regrowth. I can’t find any science on that suggestion.
With old age there is a shortening and thinning of some hair strands in addition to the lack of pigmentation. Many hair follicles where the hair is created stop working in old age and so the hair is not replaced when it is shed. Sometimes elderly cats can develop patches of hair loss. There are other causes of hair loss not linked to age but disease.
Darkening white hair in Siamese cats
This change in coat colour is specific to the Siamese and associated felines who have the gene that makes the production of melanin by melanocytes heat sensitive. The classic white centre section of the Siamese cat and dark, seal-coloured extremities (for the seal point Siamese) starts to change as the white areas become progressively darker. The contrast is lessened. Elderly Siamese cats have brown bodies and near black extremities.
This is because the body temperature in the skin of old Siamese cats is lower than for young cats due to poorer blood circulation. This increases the production of melanin, a dark brown pigment, as the process is heat sensitive as mentioned. This is my assessment. And so, the normally near white flanks of the Siamese cat can become brown in old Siamese cats.
Below are a couple of articles in cat coats. There are many more. Please use the custom search to find them.