In this review, I explore mixed-age grouping in primary schools, illustrating, through a review of scholarly research, its position within current education paradigms and in the field of education research. I justify my investigation into this topic and explicate my literature search procedure, considering the difficulties around establishing consistent terminology in mixed-age research. I explain various circumstances that give rise to mixed-age groupings and propose using four circumstantial categories – default, by-product, mandate, and preference – as a conceptual framework for understanding mixed-age phenomena. I then summarize findings from methodologically diverse inquiries into the effects of mixed-age grouping. These studies, conducted over the last sixty years, focus on many forms of mixed-age groups from around the world and consider both academic and social outcomes. Broadly speaking, outcome-based findings are inconsistent across time and place. Systematically measured differences are often small or non-existent. In the context of ambiguous empirical findings, I discuss the perspectives held by parents, teachers, and researchers around mixed-age grouping and highlight limitations of utilizing findings from comparative studies to inform education practice. I position the outcomes of my reviewed literature within the proposed circumstantial framework and discuss the implications of this standpoint. I deconstruct arguments for and against mixed-age grouping by posing the question “to mix or not to mix”, offering apparent reasoning for each position. I extend my perspective on the future of mixed-age research, focusing on the need for thorough description and clear definition of all investigated mixed-age groups, and conclude by critically considering mixed-age grouping as a promising education reform.