This paper explores the trend, between 1905 and the late 1920s in UK and US child psychology, of "discovering," labelling and calculating different "ages" in children. Those new "ages"--from mental to emotional, social, anatomical ages, and more--were understood as either replacing, or meaningfully related to, chronological age. The most famous, mental age, "invented" by Alfred Binet in the first decade of the century, was instrumental in early intelligence testing. Anatomical age triggered great interest until the 1930s, with many psychologists suggesting that physical development provided a more reliable inkling of which grade children should be in than chronological age. Those ages were calculated with great precision, and educational recommendations began to be made on the basis of these. This article maps this psychological and educational trend, and suggests that it cultivated a vision of children as developmentally erratic, worthy of intense scientific attention, and enticingly "puzzling" for researchers.