Yes, there's certainly a definite limit to how long any living cell will last in absence of opposing stressors (though hormosis has some VERY interesting thoughts on this matter - highly recommend reading up on this theory as it's gaining a lot of traction and makes some logical sense). But life and even our discrete selves are not comprised of one set of cells that last our lifetimes (some yes, but most not). Instead our cells divide and create (mostly) identical clones throughout our lives without many perceived effects in the short term, even if obvious over long term.
The true question is why doesn't this happen perpetually? If we believe that DNA is effectively the blueprint for life, then beyond the minute minority of escaping and persistent errors and external effectors to this process, then why doesn't it continue indefinitely? There's no really strong hypothesis here but some theories about the mechanics of why this occurs have been ventured. Specifically, a lot of research has been centered on the DNA telomere, which is sort of like the opposite of a rattlesnake's rattler that grows with each shedding except that it seems to be slowly lost over successive generations of cells and it's not clear why. However, there are some exciting discoveries in this area, including an enzyme which is the FDA trial I previously alluded to - as well as a supplement company already producing a version at very high cost right now. However, it's still very early and does nothing without numerous trials - and ironically will only yield insight when the subjects expire, which one would hope occurs only after receiving some of the benefits though all but guaranteed to be the opposite as par for the course of scientific pursuit. That's soon going to be the new dilemma - is the significant lengthening of one's lifespan worth the early adopter risk that it not only fails to work at great expense, but actually exacerbated and expedited that which it sought to prevent. Then again, that perspective is very much shaped if you're on the way in our the way out I guess
The true question is why doesn't this happen perpetually? If we believe that DNA is effectively the blueprint for life, then beyond the minute minority of escaping and persistent errors and external effectors to this process, then why doesn't it continue indefinitely? There's no really strong hypothesis here but some theories about the mechanics of why this occurs have been ventured. Specifically, a lot of research has been centered on the DNA telomere, which is sort of like the opposite of a rattlesnake's rattler that grows with each shedding except that it seems to be slowly lost over successive generations of cells and it's not clear why. However, there are some exciting discoveries in this area, including an enzyme which is the FDA trial I previously alluded to - as well as a supplement company already producing a version at very high cost right now. However, it's still very early and does nothing without numerous trials - and ironically will only yield insight when the subjects expire, which one would hope occurs only after receiving some of the benefits though all but guaranteed to be the opposite as par for the course of scientific pursuit. That's soon going to be the new dilemma - is the significant lengthening of one's lifespan worth the early adopter risk that it not only fails to work at great expense, but actually exacerbated and expedited that which it sought to prevent. Then again, that perspective is very much shaped if you're on the way in our the way out I guess
EDIT: grammar and typo edits