well we age because of cell deteoration...oxygen is poisonous, even though we need it. We age because telomerases on our DNA in every cell get shorter...more mistakes thus are prone to be done. Also we age because through ages our body gets a wear, just like cars get a wear. And at some point of time the whole system of organs starts failing one by one...because a heart may have stopped from clogged arteries or collapse of muscle...which leads to a domino-effect, all other organs fail and so on...and thats what we call death because the brain dies as well.
Wow, that kind of surprised me. Good answer, draqon.
There's another tantalizing explanation for why we die. I think it was proposed by Peter Medawar sometime in the 70s. He suggested that all of the ill effects of old age, that is, weaker muscles and bones, loose skin, hair loss, dementia, loss of motor control, etc., are the effects of the accumulation of sublethal genes. By sublethal gene, I mean a gene that doesn't necessarily
kill you, but increases the probability of you dying.
The idea is that genes don't care much whether a person lives forever once he has reproduced. So as a result, the selection of genes for this or that trait is weak after a person has reproduced. This is why genes like the ones causing Huntington's Disease exist. Huntington's Disease usually "turns on" when a person is in his 30s, an age when most people have already reproduced, so it's easy to see how that lethal gene can be handed down to offspring.
The other idea behind this theory is that the vast majority of mutations are deleterious. And as explained, there is significantly less selection pressure going on after a person reproduces and it continues to lessen as the person continues to age. So as a result, because there's little selection pressure against sublethal and deleterious genes, they accumulate and accumulate. They're always there, but they're not always "on"; they turn on or at least express themselves in a pernicious way later on in the organism's senescence, just like does the gene for Huntington's disease. In fact, it may even be true that these genes which later on gang up and kill us were genes that helped us in our youth, maybe coded for eye pigment or who knows? It happens though, because what an organism does in its old age isn't as important to its genes as what it does in its youth.