There are many theories of why we age, Curious we accept the ageing of a car or some tool more than ourselves. We accept growth but decline and its anticipation of death is less attractive. More personal perhaps.
Theories of ageing include the accumulation of toxins produced by gut bacteria (curable by eating yoghurt) and reduced secretions from the testicles (curable by transplants of testicular tissue from monkeys. ).
Jan Vijg has written a book Aging of the Genome: the dual role of DNA in life and death published by Oxford University Press Reviewed by Linda Partridge Nature 2007, vol 447, pp 262-3
The current accepted theory is that ageing is caused by the accumulation of random alterations to DNA in somatic tissues (all tissues other than the reproductive germline cells). What is not known is the damage responsible for functional impairment and death, and the processes that generate this damage and protect against it. Though the usual culprits ,( smoking reduces life expectation by 11 years, excess alcohol, weight , blood pressure and poor diet) must get a look in .
DNA is being constantly bombarded with chemical and physical challenges that induce random alterations, including structural damage and changes in nucleotide sequence and organization. But unlike proteins and lipids, the damaged DNA cannot be simply broken down completely and remade, because it holds unique information. Instead, cellular pathways detect alterations and dependent on the type of cell and the nature of the changes, this variously leads to DNA repair, arrest of the cell cycle (preventing cell division), cellular senescence or death, or toleration of the change. In some cell types, some forms of DNA alterations accumulate with age, with evidence for genomic hotspots and considerable variation between individuals. Cancer is a clear case where DNA alterations can give rise to age-related pathology; their role in other aspects of functional decline is less clear, with the exception of mutations in DNA within mitochondria, the organelles that power cells.
Vijg : Aging of the Genome: the dual role of DNA in life and death published by Oxford University Press Reviewed by Linda Partridge Nature 2007, vol 447, pp 262-3

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