There is symmetry to our physical lives: we are independent and robust in youth and middle age, but dependent and frail in infancy and old age. On the surface, cognition appears to follow the same general pattern of building up and wearing down. In the brain, too, the consolidation of networks in infancy and early childhood is mirrored by the reduction of connectivity and structural atrophy in older age (Box 1). In all these cases, there is a vulnerability in youth and old age that is not present in the middle of life. However, the conclusion that cognitive aging is Âdevelopment in reverseÊ is an oversimplification of a dynamic that unfolds over the lifespan, fuelling the changes that are reflected in distinct types of cognitive ability at different times of life. In the brain, for example, similar behaviors in older and younger adults are often mediated by different neural circuits [1,2]. Our purpose in this article is to propose a framework for examining changes in cognition over the lifespan and consider the implications of that framework for conceptions of cognition and the factors responsible for its change.