Does probability ensure that you will die, no matter what, once you are old enough? Does it throw the ultimate spanner in the works of life extension? The answer is not as clear-cut as you might think.
Recently, a study from Sapienza University in Italy has revived the idea of the so-called “mortality plateaus”—the apparent flattening of mortality rates in people aged above 100, suggesting that the maximum mortality rate of such people is 50% at age 105 . However, even if this mortality rate remained constant for as long as you lived, you’d still be overwhelmingly likely to die relatively soon.
What may be even more disheartening is that, if you had a constant, larger-than-zero probability of dying, then no matter how small it was, you’d be overwhelmingly likely to die past a certain point in time, be it even billions of years into the future. As a matter of fact, over an infinite time, that probability would be exactly 100%. However, the situation is not as dire as it seems.
Mind your assumptions
Your mortality rate—crudely speaking, your chance of dying at some point between two of your birthdays—goes up as you age. Assuming that you live in an industrialized country, it starts very low; you’re very unlikely to die young. However, it gets really high when you’re old. This is no surprise: as you age, more and more insults to your health accumulate until the next insult hits the threshold of what your body can take. Traditionally, mortality rates have been thought to keep going up, growing exponentially with time. The existence of mortality plateaus would imply that, for some reason, this growth stops after a certain age, and your mortality rate stays more or less constant. According to this study conducted on Italian supercentenarians, mortality rates plummet to around 50% and stay pretty much constant.
In Nature’s coverage of the study, it is said that, if mortality plateaus exist, then “[…] someone like Chiyo Miyako, the Japanese great-great-great-grandmother who, at 117, is the world’s oldest known person, could live for years to come — or even forever, at least hypothetically.” With a constant 50% chance of dying within a year, the odds that a 105-year-old person might live to age 200 are hysterically small—let alone the odds she might live forever. In fact, given this mortality rate, our hypothetical 105-year-old has only a 1-in-1024 chance to live even to age 115.
However, this mortality rate does not account for the development of rejuvenation biotechnology.
Rejuvenation and mortality rates
Your mortality rate does grow as time passes, and it strongly correlates with your chronological age, but it would be incorrect to say that your mortality rate depends on your chronological age. Rather, it depends on your biological age—in other words, how much damage the aging processes have caused to your body thus far. If, for some odd reason, mortality rates actually depended on chronological age, then we wouldn’t observe any significant diversity in lifespan of different individuals (or even species).
For now, as we have no really effective way of undoing age-related damage, your biological age will go up with your chronological one, and so will your odds of dying. However, the point of rejuvenation biotechnology is precisely to undo age-related damage; if this technology is devised and works as intended, then your biological age (i.e., the damage your body has accumulated), and thus your probability of dying of age-related diseases, will go down by some measure. How much it will decrease depends on how effective the therapies are; the goal of a truly comprehensive rejuvenation platform would be that of lowering your odds of death to those of a 30-year-old, regardless of your age, but there are caveats.
The first is that hardly any therapy will be 100% effective. You can rest assured that first-generation rejuvenation therapies will work only marginally, even though it’s equally reasonable to expect significant improvements with subsequent generations.
The second is that, as said, a miniscule probability of dying will kill you eventually if it is constant (or, more technically, if it doesn’t decrease quickly enough); in other words, if we eventually manage to reduce your probability of dying of age-related diseases to such an extent that, during any given year, it becomes only one ten-thousandth of a percent, your odds of dying of one of these diseases will eventually get very, very high—assuming nothing else killed you first. In order to avoid age-related death altogether, its probability would have to decrease fast enough with the passing of time (or be exactly zero, which is probably less likely).
What about living forever?
Age-related diseases aren’t the only cause of death; assuming that aging were taken out of the picture entirely, your lifespan would likely go through the roof, but there are other things that might well kill you—infectious diseases, weapons, accidents, wars, the possible death of the universe, and so on. The same rule of thumb applies: unless your chances of dying decrease fast enough with the passing of time, you’re statistically granted to check out at some point. Such significant mitigation of one’s risk of death from all possible causes is well beyond our current abilities (e.g., no one, to my knowledge, has an emergency plan for supernova explosions, let alone for the death of the universe); whether any of this will be possible in the future remains to be seen. So, if you were to ask me if we will ever be able to live forever, I’d play it safe: probably not, but I simply don’t know for a fact.
If you’re not afraid of a little math, you can find more details about this in an old post on my own blog.
 Barbi, E., Lagona, F., Marsili, M., Vaupel, J. W., & Wachter, K. W. (2018). The plateau of human mortality: Demography of longevity pioneers. Science, 360(6396), 1459-1461.