I realize that newer cameras, even with the same model number, may produce different white balance, saturation, contrast, etc., even using the same settings due to differences in firmware, or even hardware changes made by the manufacturer over time.
But what isn't clear to me yet, in this thread, is whether the OP is actually seeing a degradation over time in his older cameras, or if he is just noticing a difference between what his older cameras produce versus what newer copies of the same model are yielding.
If it is a degradation in the image quality, over time, then it would be interesting to find out if the sensor and/or lens may have gotten a film of dust or haze from decomposition of plastics, etc., that could be cleaned off, or if it is the sensor itself or some of the other electronics that may have degraded.
It's not as much of an issue with today's DSLRs or MILCs, but back when I first got a Canon 20D, sensor dust was a huge problem. The sensor needed frequent cleaning here, where it is dry and dusty.
A film of vapor-deposited residue from plastics, etc., on the sensor or lens surfaces will result in an overall lack of contrast, and dull colors in the images. And many plastics will off-gas some plasticisers over time, especially in hotter conditions. In a weather-sealed enclosure, you can get a film from this on everything.
I also always wonder how a security camera's sensor will hold up when it is mounted such that it receives direct sunlight (sun in the field of view) during parts of the year. But that's probably a question for a different thread.
