I know the wrong answer to this question, at least. It is “until your natural death.” The facts of human senescence and death are accidents about us set by evolution, and evolution is not our friend. The notion that there’s something good about letting nature take its course in our lives seems to be implicitly religious — it is a form of submission to the will of God. Very foolish, since nothing can answer to the concept of God. Or perhaps its some sort of ethico-mystical notion deriving from some vague sense that “nature is good.” Equally foolish. Nature is not good.

Inevitable natural senescence and death make life even worse that it would otherwise be. Here are just a few reasons why:

The knowledge of our inevitable decline and involuntary death hangs over our lives, and most people are terrified of death. Old age is often pretty miserable physically. Even crypto-theocratic bioconservatives admit this. The fact that we are eventually going to be unhealthy and frail means we spend more time in our prime at unenjoyable work than enjoyable activities, nervously saving for retirement for fear that otherwise we will be compelled to endure an penurious and undignified old age. The comparative shortness of our lives and the fact of our eventual decline means we are risk-shy about things that might be worth trying. Consider two life paths: Risky and Safe. Risky means trying for some sort of career at something that might actually be pretty interesting and fulfilling if you spend many years in your youth working hard at it — musician, philosopher, poet, etc. — but at which relatively few succeed. Safe means trudging through life in some reasonably remunerative but unenjoyable work. If you choose Risky, you are likely to end up in the position of being 30 and at the bottom of some career ladder because your plans failed, as many will. So your middle life will be even worse than it might otherwise have been, as you scramble desperately to make something of yourself before decline sets in — you suffer from (3) but even worse than other people. (I know folks — too many — who find themselves in this position.) But if you choose Safe, you get to live out your life in regret at what might have been.

If not natural life, then what? There are two possible answers that strike me as reasonable and principled.

Only as long as it takes you to end your life. This is an argument that I covered before: if people have temporal parts, then it’s in your hands whether there are future selves descended from you. Those selves will suffer and will have frustrated desires. You should prevent this.

Now perhaps this position is unsound. Maybe people don’t have temporal parts, or maybe David Benatar’s propostion that there is an asymmetry between good and bad things in people’s lives is mistaken. Or perhaps, because the argument only leads to a pro tanto reason for suicide, the reasons for ending one’s life are outweighed by other reasons. In any of these cases, we might want to consider an alternative possibility.

You should have healthy life as long as you want, and end your own life when you’ve had enough. This is science fiction, of course, but respectable science fiction: some version of suicide is the principle (almost only) reason why sentients routinely die in stories like Greg Egan‘s Diaspora or Cory Doctorow‘s Down and Out in the Magic Kingdom. I don’t think that such a state of affairs is technically unfeasible, though it might be a while before it does become possible.

This last state of affairs could make people’s lives much better, even if they weren’t otherwise improved. Not only would they not be subject to the kinds of misery detailed in natural life under 1-4 above, but they wouldn’t have to experience regret at things left undone, assuming they really want to do them.