The increasing age of the population around the world has meant that greater attention is being paid to disorders that mainly affect older people. In particular, work is focusing on ways to preserve the healthy brain and prevent dementia. Preventive studies are complex and must take into account not only simple approaches such as those used in risk and outcome studies, but also stage of life, survival and mortality, and population context before their effect can be assessed. This paper presents questions and areas which must be explored if the potential for prevention of dementia during brain ageing is to be properly understood.