The world's population is aging, which will result in an increasing prevalence of neurodegenerative diseases, such as dementia. Observations from functional brain imaging that older brains can be more active than their younger counterparts challenge stereotypical ideas of aging. In those aging successfully, brain activation is more anterior, less lateralized and more coordinated than in those at risk of, or suffering from, cognitive impairment. Several theories have been proposed to explain these findings. One of the most enticing is the scaffolding theory, which posits that the older brain is a plastic homeostatic organ, able to compensate for its deteriorating structure. However, with aging also come diffuse vascular changes and the resulting white matter damage. This decreases the compensatory capacity, and dementia can ensue. This and alternative hypotheses will be discussed, along with potential methodological problems of this genre of study and with their clinical implications.