Here is the actual reason we grow old and die. Are you ready? It’s brutal:
Let’s say there is a 2% chance that any given primitive human will die from external events each year. He might get eaten by a bear, starve to death, have a bad fall and get left behind by his tribe, get clubbed in the head, lost in the woods, etc. We can get the probability of not having been killed by an external event as (chance of being alive)^(age)
For example, at age 10 that is .98^10=81%, age 30 is 54%, 70 is 24%, 100=13%.
For any given unit of energy and nutrition, the genome can either invest in current reproductive potential or preserving the body for future reproductive potential.
For a 10-year-old, his reproductive potential is in the future so the body will invest most resources in self-maintenance.
For a 30 year old, the genome will balance the energy directed to self-maintenance and for preservation for future reproduction.
Since most humans will die by 70 due to an external event, the self-preservation mechanisms will not optimize for either.
The genome just doesn’t “care” at this point, since most of the hosts carrying its instructions are already dead. That’s when (and why) the body begins to rapidly fall apart.
One key mechanism for this process is in the cellular tumor antigen p53, mutations of which cause the majority of cancers in humans. There is a tradeoff between anti-cancer mechanisms and energy expenditure since cell-malfunction detection uses energy. Species which obtain an evolutionary benefit from long lives have much better cancer-prevention mechanisms. That’s why the naked mole rat (which lives in underground colonies) is resistant to all forms of cancer and can live for many decades while mice (which are food for many predators) live for two years.
So, the reason that we die from old age is that your genome expects you to be dead from an external cause after a few decades and does not care about keeping you alive.