Compared to the large body of research into gender, race and class in children’s literature, there has been little awareness of the social construction of age in this discourse. Analysing age in contemporary fiction for young readers gives insight in how present-day society models (people of) different ages, and given the decisive role that books play in shaping children’s worldviews, such research contributes to our understanding of how age norms are passed on across generations. This article explores the construction of age in J.K. Rowling’s Harry Potter in relation to the age of the implied reader. This case study provides a unique opportunity to study age, because the main characters in every volume ‘grow up’ together with the implied readers. This article traces the correlation between the evolutions in form and content in J.K. Rowling’s Harry Potter series on the one hand and an evolution in the age of its implied readership on the other. After scrutinising existing guidelines pertaining to the ideal age at which to read each book, we conduct our own digital analyses on the style and topics of the texts. As well as providing insight into the evolution of these features in the Harry Potter books, this article contributes to the ongoing discussions on the reliability of readability measures and the desirability of explicit age markers on books for young readers.