A growing body of research using both behavioral and neuroimaging data points to a significant effect of bilingualism on cognitive outcomes across the lifespan. The main finding is evidence for the enhancement of executive control at all stages in the lifespan, with the most dramatic results being maintained cognitive performance in elderly adults, and protection against the onset of dementia. A more complex picture emerges when the cognitive advantages of bilingualism are considered together with the costs to linguistic processing. I will review evidence for both these outcomes and propose a framework for understanding the mechanism that could lead to these positive and negative consequences of bilingualism, including protection against dementia in older age.