There are lots of unknowns, but we do know that oxidative damage is a factor and can be avoided [or even reversed](https://youtu.be/QRt7LjqJ45k?t=421).
Cell repair, complex functions, and mitosis all require DNA be unraveled from the chromosomes, during which time it’s vulnerable to oxidizing agents (oxygen, nitrogen oxides, and certain acids) and radicals (generally singlet oxygen which is produced by the immune system) which cause changes such as methylation – addition of a methane molecule where there shouldn’t be one – which blocks proper transcription, causing breaks in protein chains, or even failure of stop/start regions in the code that cause run-ons or complete failure of that cell’s genes to produce that protein sequence.
Well, that’s asking for an unknown. There are a lot of theories about this, ranging from telomeres running too short effectively repair DNA (the surplus DNA material at the end of your chromosomes), to radiation of all sorts (not just radioactivity) effectively damaging cells as they multiply and causing the damaged cells to be the new model for further mitosis (cell division).
As far as I can tell, there is currently no “known” answer to your question, only difficult-to-prove theories.
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