Legislation for dementia care needs to be continually rethought, if the rights of older persons and other persons with dementia are to be addressed properly. We propose a theoretical framework for understanding vulnerability and dependency, which enables us to problematize the currently prevailing legal conception of adults as always able — irrespective of health or age — to act autonomously in their everyday lives. Such an approach gives rise to difficult dilemmas when persons with dementia are forced to make decisions on their own about basic living conditions, such as housing and care, without decision-making support. In Sweden, for example, such matters are frequently left to the person him- or herself to decide, often without any assistance from social workers, and with family members serving as caregivers of last resort. Using vulnerability theory as the framework for our discussion, we argue that policymakers should not apply a group-oriented approach (based on factors like age, legal status, or mental capacity) to persons suffering from dementia. The needs of such individuals are as complex and varied as they themselves are. We discuss our findings from an interdisciplinary (law/social work) research project in which we examine the dilemma that social workers face when they are required, under the terms of the Swedish Social Services Act, to determine whether persons with dementia are to be granted support. We argue further that a cross-disciplinary approach — in which vulnerability theory furnishes the framework — opens up for new ways of understanding and developing social welfare law and practice. This, we believe, can help us better address the rights, interests, and needs of people with dementia, of their families, and of professionals in the social welfare system. Finally, many of the problems faced by ageing societies in general can be understood on the basis of such an approach.