Genes provide instructions for developments in the body. As the body grows and ages (which is a consequence of gene activation which in turn leads to the production of growth hormone and hormone production) different genes become activated. This is called regulation of gene expression and concerns mostly genes related to protein instruction and production.
When we are fairly sexually youthful say below 30, our bodies are at their best. Our immune systems are at their strongest, our muscles and bones are at their peak and our fertility rate is at its highest. Many of our genes are expressed in our childhood, teenage and young adult years. These genes seem to be good, i.e. they seem to enhance our bodies. However this is not ingrained in mammals. It is not biologically a necessity that our good genes should express themselves at the beginning of their lives.
Evolution has made it so, over time, it seems that any 'bad' genes which may have been expressed early on in our lives have been sieved out. As these genes are deemed 'bad' which means they are detrimental to an individuals life span, individuals with these genes would have trouble reproducing or not reproducing at all.
http://publications.cancerresearchuk.org/downloads/Product/CS_DT_INCMORTRATES.pdf (Mortality rates for cancer... e.g. prostate cancer carriers to death ratio ~ 106:28. Over time in youths, this would have a dramatic affect).
These 'bad' genes could activate cancers or permutations in the body which could limit its survival. Those without these genes would live longer and reproduce more. Over time (a long time) this would result in people not having these bad genes, as they would fail to reproduce at a significant rate, obviously a few still have these genes expressed early on but it is rare.
Reproduction obviously occurs quite young, so genes which are activated after reproduction (in females) and after a certain age in men could be bad. As those individuals carrying it would not be at a reproductive disadvantage, i.e., in their youth they would be healthy and reproduce the same amount as other individuals.
Genes are expressed due to chemical cues in the body, so the presence or lack of a hormone or chemical could activate a gene. Through time and after reproduction our bodies could stop producing the same amount of a hormone, say, progesterone (in females) which would be a cue for a 'bad' gene to start expressing itself.
So, it could be argued that the activation of these 'bad' genes (which could be mutations or copying errors too) could by themselves result in aging.
This is an extremely sly way for unwanted genes to persist through evolutionary time... they get passed on through reproduction because they haven't been activated yet. So reproduction has led our species into a happy enough youth but the age at which we reproduce(d) at has resulted in a bad old age, as we have not been able to sieve out the bad genes.
Obviously technology and medicine has helped kurb bad genes, however we can go further with this and find the heart of how these genes become expressed in those ages and maybe result in a longer and better quality old age.
(This is basically Peter Medawar's theory on aging using mutation accumulation)
See Also: http://newphysicistphi.blogspot.co.uk/2012/08/survival-machines-worshiping-gene-part-i.html
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