Over and over again, someones says that living for a very long time would be a bad thing, and then some futurist tries to persuade them that their reasoning is faulty.  They tell them that they think that way now, but they'll change their minds when they're older.

The thing is, I don't see that happening.  I live in a small town full of retirees, and those few I've asked about it are waiting for death peacefully.  When I ask them about their ambitions, or things they still want to accomplish, they have none.

Suppose that people mean what they say.  Why do they want to die?

EDIT: Incorporated suggestions from comments: Moved off-topic parts into comments, improved formatting, corrected links.

Definition

The LW post Value Deathism differntiates between the illusory nature of death and the 'desirability' of death called deathism proper. This post is about the latter. Where desirability is meant in a general sense and not (only) in the sense of desirable for an individual.

I propose a different more neutral term for deathism: Senexism - from the latin adjective senex - old.  I propose this because death is only the end of an aging process and by focussing on the ultimate and emotionally disturbing result one loads the topic with negative connotations. Senescence on the other hand - though unwanted - has also positive connotations of experience and humility. This also nicely splits off (or reduces applicability of) death by accident. 

Outline

My defense is twofold. First I address the (emotional) pain and loss death causes and point out adaptive affects of the coping mechanisms humans have. Second I address the actual benefits senescence and death has - not for the individual but for the group. Thus the latter is an utilitarian argument for death actually.

I will provide current research results for these points. At the end I will conclude with an opinion piece on what this means for rationalists and an outlook how this applies in light of the singularity.