This article reconsiders the early critical reception of Algernon Charles Swinburne’s 1866 collection Poems and Ballads with a view to articulating the extent to which the critical hostility that famously greeted the book upon publication was mediated by the category of ‘boyishness’. I show that the complaint that the 29-year-old Swinburne wrote, and by implication thought and felt, too much like a boy and not enough like an adult man lay at the core of the critical onslaught and contributed to underpin critics’ various complaints of obscenity, blasphemy, bad taste and so on. After considering the nature of the connection between the boyish quality often associated with Swinburne as a person throughout his life and the poetical ‘boyishness’ critics perceived in his work, I propose a taxonomy of three main meanings of boyishness that emerge from the early critics’ attacks: boyishness as lack of virility, boyishness as lack of self-restraint, and boyishness as lack of intellectual maturity. By analysing these critical readings in the context of various medical, pedagogical and more broadly cultural discourses of the time, I make the case that Swinburne found himself cast as someone who presented precisely the characteristics of boyhood of which a functioning adult man was supposed to rid himself. The broader argument is that by giving close attention to age-based slurs, we can gain a more fine-grained account of mid-Victorian attitudes to childhood and maturity, and society’s self-image more generally.