Some while ago Harpshot commented on the fact that the body was constantly renewing itself, and went on to the problem of ageing.  I wanted to comment on his remarks (below): 

I was amazed (some years ago) to learn that one’s body is constantly re-building itself. Cells just don’t live very long, it seems. Thus, the critical role of one’s personal genome, or DNA strands, in assuring that proper cells are constantly and consistently built to replace the ones that die. This helped me understand, a little better anyhow, things like cancer and other medical issues. It also helped me understand what I’d heard once upon a time about the mystery of aging. A person dying of “old age” is made entirely of cells that are practically brand new! So how does a collection of brand new cells make an “old” body? Maybe it’s like making a copy of a copy of a copy… It keeps getting worse to the point of illegibility. It does seem to raise immortality from pure fiction to the realm of possibility – just find the design “flaw” and fix it. Strangely, when I ponder the question of whether there’s “divine intelligence” behind existence, I find this to be the strongest evidence in support thereof. Some people point to the wondrous beauty and complexity of creation as proof of “intelligent design”, arguing it’s beyond the possibilties of random combination of atoms and molecules over time. Personally, I find it hard to believe any intelligence, however omnipotent, could accomplish such a feat as creation of the universe. But, I find no good reason for aging (an inevitable death) other than intentional design. On the other hand, maybe I got that stuff about cells all wrong. My memory ain’t so good as it used to be.

Robert’s comment:

I owe the following to The Selfish Gene by Richard Dawkins:

One theory is that ageing represents an accumulation of harmful copying errors and other gene damage, which occur in an individual’s lifetime. Another theory (Sir Peter Medawar) is that senile decay is a by-product of the accumulation in the gene pool of late-acting lethal and semi-lethal genes, which have been allowed to slip through the net of natural selection, simply because they are late acting.  Selection has the effect of postponing the operation of deleterious genes and favouring those that have a good effect and assist in their survival.  But some lethal genes slip through, having been passed to the next generation while the individual is of child-producing age.  These late actors may be turned on by chemical changes in the human body as time goes by  (it might be possible to trick these genes into believing that they inhabit younger body than in fact they do,  if these chemicals could be identified).