Socio-economic differences in health appear to be universal but the precise pathways that link socio-economic status and health remain unclear. Differential exposure to specific risk and protective factors are often cited as, at least, partial explanations of socio-economic differences in health. However, risk factors are culturally specific and risk factor-specific models of socio-economic differences in health may be inadequate: as soon as prevailing risk factors change, so too must associated explanations of socio-economic differences in health. An alternative, risk factor-independent, model of socio-economic differences in health proposes that fundamental pathways to health and disease exist and that risk and protective factors act by feeding into these pathways. We propose that biological ageing is one such fundamental pathway to health, disease and, thus, socio-economic differences in health. Biological ageing is the progressive decline in physiological ability to meet demands, that occurs over time. It is due to the accumulation of damage at the cellular level and the rate of biological ageing is determined by both environmental and genetic factors. There is increasing evidence that many known disease risk and protective factors influence the rate of cellular damage accumulation and hence biological ageing and that the pathogenesis of some important diseases is related to biological ageing. We discuss these issues and hypothesize that socio-economic differences in health are partly a result of poor people ageing faster than rich people due to the unhealthy environments to which they are exposed.