What is the meaning of 老不死 exactly?
老不死 == 老而不死 being old, but not die; It implies some old guy should die as soon as possible, rather than stay alive, wasting food and hurting others... etc. 
It's used to curse some old people whom you hate and consider to be useless to others.

It doesn't mean Chinese think old is bad, instead Chinese people have a tradition to support old people.. Whereas, if there's someone who did nothing good when he was young, accomplished nothing when he was an adult, stays alive when old, and keeps hurting people around, his life then is considered useless and even harmful to others. You then can call him 老不死, to curse him to die early.

Also, there are some people who hate old people and think they are useless all over the word. For this kind of people, they might just use the word 老不死 to refer to any old people, in a derogatory way.

The original of the word was from Confucius. 子曰: 幼而不孙弟, 長而無述焉, 老而不死, 是為賊.