This is self-sampling assumption-like reasoning: you are reasoning as if experience is chosen from a random point in your life, and since most of an immortal’s life is spent being old, but most of a mortal’s life is spent being young, you should hence update away from being immortal.
You could apply self-indication assumption-like reasoning to this: as if your experience is chosen from a random point in any life. Then, since you are also conditioning on being young, and both immortals and mortals have one youthhood each, just being young doesn’t give you any evidence for or against being immortal that you don’t already have. (This is somewhat in line with your intuitions about civilisations: immortal people live longer, so they have more Measure/prior probability, and this cancels out with the unlikelihood of being young given you’re immortal)
This is self-sampling assumption-like reasoning: you are reasoning as if experience is chosen from a random point in your life, and since most of an immortal’s life is spent being old, but most of a mortal’s life is spent being young, you should hence update away from being immortal.
You could apply self-indication assumption-like reasoning to this: as if your experience is chosen from a random point in any life. Then, since you are also conditioning on being young, and both immortals and mortals have one youthhood each, just being young doesn’t give you any evidence for or against being immortal that you don’t already have. (This is somewhat in line with your intuitions about civilisations: immortal people live longer, so they have more Measure/prior probability, and this cancels out with the unlikelihood of being young given you’re immortal)