It is well known that immigrant fertility is associated with age at migration, but the majority of prior research has focussed on foreign-born women who migrated as adults. Much less is known about the fertility of immigrants who arrived as children, often referred to as child migrants or the 1.5 generation. This study aims to generate new insights about the role of age at arrival in determining the fertility of child migrants later in life. It carries out a case study of Sweden, using longitudinal register data on the whole population. These data allow a comprehensive analysis of completed fertility profiles, for both male and female child migrants, from age 15 to 45. They also enable the use of family fixed effects models, and detailed analysis of specific countries of birth to examine heterogeneity and the potential for generalisation. The results show that there is evidence of fertility adaptation for women and men. This evidence is not consistent across origins, and it also varies – by origin group – across different stages of the childbearing schedule. Nevertheless, the results for men are not dissimilar from those for women, including by country of birth. This may represent evidence of an underlying process, like childhood socialisation followed by adaptation, that is common for women and men. There is less evidence in support of critical ages, although some of the results suggest that this might be a useful avenue for further research.