Ultra-high-field fMRI at 7 Tesla in adults revealed that hemodynamic responses to visually presented numerosity are topographically organized like receptive fields in several cortical areas (Harvey et al., 2013). It is currently unknown, however, whether receptive-field like responses can also be detected in developing brains. In this presentation, we will report the results of a series of 7 Tesla fMRI experiments conducted with adults and 10-12 year-old children in which we presented numerosity in visual and auditory format. Developmental differences are captured by comparing numerosity maps of adults and children at the individual and the group level. We will test the hypothesis that developing numerosity maps are smaller and more broadly tuned compared to adults. Moreover, we will explore potential links between numerosity tuning and behavioural maths test performance. Our work will advance our understanding of individual differences in numerical cognition including neurodevelopmental disorders like dyscalculia.