Free radicals (such as reactive oxygen species) are increasingly generated with age - this is the end of a long chain of consequences that starts with damaged mitochondrial DNA. How do those oxidizing agents actually cause widespread harm to bodily systems? This paper gives an overview of one broad set of mechanisms, wherein step one is the creation of reactive carbonyl species (RCS) by free radicals: "Most of the biological effects of RCS [are] due to their capacity to react with cellular constituents, forming advanced lipoxidation end-products (ALEs). Compared to reactive oxygen and nitrogen species, lipid-derived RCS are stable and can diffuse within or even escape from the cell and attack targets far from the site of formation. Therefore, these soluble reactive intermediates, precursors of ALEs, are not only cytotoxic per se, but they also behave as mediators and propagators of oxidative stress and cellular and tissue damage. ... The causal role of ALEs in aging and longevity is inferred from the findings that follow: a) its accumulation with aging in several tissues and species; b) physiological interventions (dietary restriction) that increase longevity, decrease ALEs content; c) the longer the longevity of a species, the lower is the lipoxidation-derived molecular damage; and finally d) exacerbated levels of ALEs are associated with pathological states."

Link: http://www.ncbi.nlm.nih.gov/pubmed/18721793