Yes, Siamese cats become darker as they age. You’ll see this in the flanks and central areas of the cat’s body which are normally lighter and sometimes almost white, normally.
Normal situation for the Siamese cat
The reason for the cat’s “pointed” body is that the cells (melanocytes) that produce the pigment (melanin) that is delivered to the hair shafts are temperaure sensitive due to the presence of the gene cs. The Siamese cat is a black cat but for this gene (non-agouti black – i.e. plain black).
When a Siamese cat is born they are white all over as they hot. As they cool their extremeties darken because the extremeties are cooler than the center of the cat. This is because the blood carries heat and at the distal parts of the cat there are less blood vessels and the extremeties get colder in the same way that our hands and feet become cold in winter. They are exposed more to the ambient temperature (the temperature of the air around them) which will normally be lower than body temperature (100.5-102.5 degrees Farhenheit).
The cooler temperature of the skin at the extremeties results in the melanocytes producing more melanin.
Older Siamese cat
There are three reasons why their fur becomes darker in the middle of their bodies.
As the cat ages their ability to thermoregulate becomes less efficient which means they can’t maintain body temperature as well resulting in a cooling of the central portion of the body.
Also the blood vessels become narrower in the ageing process so they carry less blood resulting in the skin becoming colder. This results in the melanocytes producing more pigment which turns the flanks and central parts darker.
Thirdly, genuinely elderly cats lose some body fat which means they lose heat causing the skin to be colder resulting in less pigment production.
Shaving coat throws up interesting anamoly
Temperature controls the appearance of the Siamese cat. You can pretty well guage the age of a Siamese cat by the darkness of the central part of their coat. P.S. The advantage of the cooler body is that the cat is less likely to have fleas as the blood vessels are thinner.
Some more on pointing