Why assume there is a clock, rather than assume the damage is at the metabolic level? None of the predominant forms of human chronic disease that lead to most instances of death today (artherosclerosis, cancer, neurodegenerative disease, metabolic dysfunction like diabetes) seem like an intentional function. They seem like unintended consequences of other functions, like lipid transport or DNA replication, that don’t get selected against because they fall beyond the natural reproductive lifespan of most people. I suppose you could say that the biological clock in question is the number of eggs a woman has, but a simpler explanation for limitation could be that eggs are just very energetically expensive to produce.