There seem to be a number of ideas about why we age. Hypotheses include the gradual accumulation of cell metabolic products affecting organism function and the reduction of telomere length during cell division. My hand-wavey idea would be "wear and tear".
Are we anywhere near a consensus theory of senescence?
Answer
The 'wear and tear' argument is most likely true but it is also interesting to reason about ageing as inevitable from the evolutionary point of view.
To set up the argument, we need two things: First, each individual has got a 'reproductive potential' which is realised throughout life. This means a deleterious mutation which has an effect in early life, will affect reproductive value more than a mutation which manifests itself in later life, after the individual has already had offspring. Thus selection will act strongly on genes which are expressed in early life than on those which are expressed later. For that reason, there's no strong selection against diseases such as diabetes or cancer. This argument can be applied not only to occurrence of disease but also to decay of ordinary functions of the body.
Secondly, cells in the body are constantly renewed and defects such as telomeric breaks are repaired. Mutations in the soma are taken care of by the immune system and can be in principle avoided. The fact that they tend to accumulate in later life can be explained by the first point: selection is weaker to oppose telomeric breaks and mutations in later life.
I was trying to be brief here, but there are more sides to the argument (e.g. Williams' antagonistic pleiotropy). Modular Evolution (Vinicius, CUP 2010) provides a good overview of the evolutionary aspect of theory of senescence (and many other interesting evolutionary arguments).
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