There are a lot of theories of aging, some very useful, many of which overlap, and many of which are overly narrow, overly general, or otherwise unhelpful. Here's one that appears to be another way of looking at damage accumulation, or perhaps reliability theory: "Individual differences in the rate of aging are determined by the efficiency with which an organism transforms resources into metabolic energy thus maintaining the homeostatic condition of its cells and tissues. This observation has been integrated with analytical studies of the metabolic process to derive the following principle: The metabolic stability of regulatory networks, that is the ability of cells to maintain stable concentrations of reactive oxygen species (ROS) and other critical metabolites is the prime determinant of life span. ... Our studies delineate age and tissue specific patterns of transcriptional changes which are consistent with the metabolic stability-longevity principle. This study, in addition, rejects the free radical hypothesis which postulates that the production rate of ROS, and not its stability, determines life span."

Link: http://www.ncbi.nlm.nih.gov/pubmed/19031007