“90 years from now I’ll still be alive” (supposing arguendo that you’re 30 now) -- that has small probability either way.
This is the closest one. However, that’s not an observation, it’s a prediction. The observation is “90 years ago, I was 30.” That’s an observation that almost certainly won’t be made in a collapse-based world; but will be made somewhere in an MWI world.
“I’m alive at 120, here and now”—that has small probability either way. (On most branches of the wavefunction that include your present self, no version of you gets to say that.)
“small probability either way” only applies if I want to locate myself precisely, within a branch as well as within a possible world. If I only care about locating myself in one possible world or the other, the observation has a large probability in MWI.
This is the closest one. However, that’s not an observation, it’s a prediction. The observation is “90 years ago, I was 30.” That’s an observation that almost certainly won’t be made in a collapse-based world; but will be made somewhere in an MWI world.
“small probability either way” only applies if I want to locate myself precisely, within a branch as well as within a possible world. If I only care about locating myself in one possible world or the other, the observation has a large probability in MWI.