The developing brain is highly plastic, meaning it can change its structure and function in response to its environment. Increased plasticity early in life enables children to learn and adapt to positive and negative experiences. Individual differences in children’s potential for brain change, however, remain poorly understood. Characterizing variability in plasticity as a potential for change may shed light on individual differences in learning and response to interventions. Work in animal models has revealed cellular and synaptic factors that restrict (e.g., myelin) or promote (e.g., dopamine) plasticity, but these properties are difficult to measure at scale in humans. Neuroimaging proxies can instead measure features of the human cortex that are sensitive to factors that modulate plasticity. I leveraged such methods to ask how individual differences in plasticity change during development and how they are associated with learning. In my first study, I examined how proxies of cortical myelin vary with age in childhood. There were strong, positive age effects in early-developing sensorimotor cortices, suggesting these areas mature earlier than higher-order, transmodal areas. In my second study, I explored whether pubertal development predicts individual differences in cortical myelin beyond the effects of chronological age. Girls with earlier onset of menses showed greater sensorimotor maturation from age 10 to age 12 compared to their peers. These findings support the hypothesis that puberty causes changes in cortical microstructure and suggest that earlier pubertal timing is associated with decreased plasticity. Finally, in my third study, I tested how individual differences in proxies of myelin and dopamine predict individual differences in learning. Young adults with less myelin and stronger dopamine system connectivity, indicative of higher plasticity, showed greater learning gains following one hour of memory training. Together, this work suggests that neuroimaging tools can detect individual and developmental variability in cortical features sensitive to cellular factors that modulate plasticity and learning. My findings may have important implications for the design and developmental timing of educational and psychological interventions that aim to harness brain plasticity to optimize learning and well-being.