There is a theory of aging that I particularly like. I read about it in one of Richard Dawkins's books. He referenced it as something akin to the Sub-lethal Gene Theory. I suppose it is probably the mainstream accepted explanation for why we age because it follows so naturally from the established principals of evolution. When one fully comprehends the theory, there is a sense of inevitability, as if it can't not be the explanation for why we age. In a sentence the theory can be summed up as: Aging is caused by the cumulative effect of sub-lethal genes. There is an old adage that goes something like "You can't do one thing." A way to put it more explicitly is that when you cause something to happen the event ripples and reverberates through all four dimensions through all spacetime, even though you were probably only trying to affect something near you. This is also true of genes. Because it is so often heard, "a gene for this..." "... a gene for that," in the media, it is easy to think that individual genes have a purpose, to, perhaps cause blue eyes or red hair. But genes don't work that way. Genes aren't like components in a radio, in which it can properly be said that transistor x is for purpose y, because unlike radios, animals and organisms aren't designed. In evolution, there is no purpose. In evolution a gene naturally and automatically accumulates in an organism because it is useful on average. If a gene is harmful to an organism in one respect, but extremely helpful to the organism in another respect, then natural selection will favor that gene and it will be caught and kept by the sieve for future use. Nevermind having a single purpose, a gene has no purpose. When a scientist says "a gene for x" he means in the presence of that gene, trait x is significantly more common, nothing more. It doesn't matter how indirectly the gene causes the physical effect. And it is common that genes produce their claimed purpose or physical effect very indirectly. Gene x switches on gene y, which references genes, xx, xy and yy, which code for and produce the proteins needed to start process xxxy, which switches off gene z, which statistically, almost always causes a person to have blue eyes. It turns out that gene x does many other things, such as cause the right big toe to be slightly shorter than the next toe in line, an allergic reaction to pollin, prolonged stage of shyness during adolescence and a like for spicy foods. Like an event in spacetime, the effects of a gene can branch out and touch in long, winded and unpredictable ways. All of our genes are lucky mutations, instructions that intruded into our bodies by chance but were deemed worthy of being kept by natural selection. That, or natural selection was momentarily away and they sneakily slipped through the unguarded walls. Mutations are random and are, therefore, mostly bad and harmful. There are many, many, more ways to be wrong than right. There are many, many, more ways to not hit the bull's eye than there are ways to hit the bull's eye. Genes exist and produce their effects in spacetime. Generally, it's most important that an organism be its fittest during its youth, in order to reach adulthood, and during its adulthood, so it can reproduce and see to it that its progeny have a fair shot at life. Genes are going to be selected to make it strong and fit specially during that time. But remember that genes can't do one thing. Each gene that does something good during that time of youth still exists after its period of usefulness and since it can't do one thing, it is does other things as well. Remember that most mutations are harmful, and pretty much, the rest of the story is told. The genes' usefulness are used up during youth; it's not important that they cause the organism to be strong during old age. As the organism gets older, increasing numbers of normal and helpful genes accumulate to defect against the organism by veering off in different directions and branching off in unpredictable ways. They fall out of phase with the organism. The result is a gradual accumulation of ill effects: weaker bones, poorer eyesight, deterioration of the brain, loss of muscle tone. A general and inexorable collapse of the edifice. The older you get, the more the genes veer and appear to not care. If you don't die of this, then you'll die of that. When you get past a certain age, I suppose, it becomes misleading to say He died of cancer or heart disease. Really, it's old age.