Separated by almost three hundred years and by significant developments in the construction of childhood as an identity distinct from adulthood, the early seventeenth and late nineteenth centuries stand out as periods of intense and popular activity by child actors, and specifically by companies of child actors who played adult roles and deliberately juxtaposed the age categories of performer and character. While much excellent period-specific work has been accomplished during the last twenty years on early modern boy actors and on the ‘infant phenomena’ of the Victorian stage, there has as yet been no attempt to compare children’s professional performance during these two periods. This article contrasts the repertoire of the boy companies of Jacobean London with that of the children’s opera companies who toured the UK and Ireland throughout the 1880s performing comic operas such as H.M.S. Pinafore and Les Cloches de Corneville. It also explores what it meant for children to perform as adults during these two periods, and what the latter reveals about the historical construction and policing of age categories.