Disruption of the balance between excitatory and inhibitory neurotransmission (E-I balance) is thought to underlie many neurodevelopmental disorders; however, its study is typically restricted to adults, animal models, and the lab-bench. Neurophysiological oscillations in the gamma frequency band relate closely to E-I balance, and a new technology—OPM-MEG—offers the possibility to measure such signals across the lifespan. We used OPM-MEG to measure gamma oscillations induced by visual stimulation in 101 participants, aged 2–34 years. We demonstrate a significantly changing spectrum with age, with low-amplitude broadband gamma oscillations in children and high-amplitude band limited oscillations in adults. We used a canonical cortical microcircuit to model these signals, revealing a significant decrease in the ratio of excitatory to inhibitory signalling with age in the superficial pyramidal neurons of the visual cortex. Our findings detail the first MEG metrics of gamma oscillations and their underlying generators from toddlerhood, providing a benchmark against which future studies can contextualise.