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Over the next thirty years, the increase in life expectancy will dramatically change the demography of industrialized countries. For the growing number of elderly people, the challenge is not only to live longer, but also, and more importantly, to delay the damaging effects of ageing on cognitive and motor abilities that alter individual autonomy. It is now demonstrated that physical activity exerts protective effects that promote the formation of a cognitive reserve. In this article, we propose a model of ageing and a model of the characteristics of physical exercise. We then briefly describe the effects of exercise on brain plasticity and cognitive functioning. Finally, we develop different assumptions about the mediating mechanisms that predict and understand the effects.