Internationally, schools with small numbers of children and constrained funding are not able to group the children in classes based on their age. Rather, the children are combined into larger ‘mixed-age’ classes. Schools and teachers manage this situation in different ways, for example by teaching the class as two or more separate age-based groups within the classroom, or by ‘mixed-age’ teaching of the whole class together. All teachers face the challenge of diversity within their class of learners but arguably this is exacerbated for teachers of mixed age classes. In England, this issue of mixed age teaching has been foregrounded during current efforts to reform the teaching of mathematics, where a national policy is focused on developing ‘teaching for mastery’. The characteristics of mastery approaches to teaching mathematics are contested and there is considerable variation between schemes and schools. Mastery approaches developed in England have been influenced to different degrees by practice in East Asian nations including Singapore, Shanghai, and Japan, which have all, with their varied contexts and approaches, seen success in international comparative tests. This paper provides a selective review of international research into mixed-age teaching, judging the current evidence to be of some use but insufficient, and positions the findings in relation to key characteristics of mastery approaches to teaching mathematics. Three generally agreed characteristics of mastery approaches to teaching mathematics, expectations of success for all pupils, in-depth sustained study of topics, and whole class teaching, appear to offer opportunities as well as additional challenges for teachers of mixed-age classes.