Background: Numeracy development is widely accepted as critical for adults in the general community which is equally the case for adults with Down syndrome. This paper reports some of the findings of a study including the research question: What is the justification for numeracy development for adults with Down syndrome? Investigating this question led to the search for a framework to support the ongoing development of numeracy. Method: The research used a case study methodology. Five adults were observed and interviewed in two contexts each. The data were analyzed to identify links to the quality of life model. Results: The research illustrates how a quality of life approach can be used to justify and guide the lifelong development of numeracy. Data from the case studies linked numeracy to quality of life under the principles of personal contexts, variability, life-span perspective, values, choices and personal control, perceptions and self-image. Conclusions: The principles of quality of life can provide a framework for the development of numeracy in the context of adulthood and Down syndrome. Preparation for the numeracy needs of a long and satisfying adulthood should begin in early childhood, continue in schools with the teaching of underlying mathematics concepts and skills, and be modified and refined throughout adulthood by the use of a numeracy development plan. Carers and professionals interacting in the contexts need to adopt a teaching role for numeracy.