They can deteriorate over time from Cosmic Rays. It would be random pixels that do not work anymore.
This PDF article was written in 2001 and explains what happens.
EDIT: In my experience its the support electronics that fail from corrosion of the printed circuit board tracks. Dew eventually finds its way into the camera and anything metal turns to rust.
I have a 12 year old QHY8 and a 10 year old QHY9. Both developed hot pixels and bad columns over time. It also seems to accelerate with age. Still good to use.
I'd say it was not a significant issue with CCD's.
I used ATiK 16ic cameras for many, many years with no deterioration.
My ATik 314L again was used for many years (before failing!) with no issues.
Just my 2c