We adopted a 4 year old, Ashley, back in 1990. He and his brother were brought to the shelter after their owner died. The brother was adopted in 24 hours after being in the shelter. Ashley languished there for about 8 weeks. Then I came in the door. The shelter manager told me that she NEVER saw a cat as depressed as Ashley was, and she said if he was a human, he would be considered suicidial!!! He did hide under the bed for the first three days when he came home, then Ashley adjusted.
I think any cat coming from a home who is put in a shelter would get depressed because the cat does not understand why it is there, and it is a stressful place with all the different cats, dogs, food, and living in a cage. I can see why some older cats just give up and die, probably of a broken heart.