This is an exciting time for scientists who are interested in cognitive development: there is now a wealth of easily-accessible data that can be used to ask interesting questions about how psychological, neural, and genetic factors affect changes in cognitive functions across the lifespan - and how they differ between individuals. In this talk, I'll describe several studies that apply individual-differences methods to large-scale, sometimes longitudinal datasets that include cognitive and biological information. I'll start by covering the uses and limitations of polygenic risk scores in predicting cognitive abilities early in life. I'll then discuss how factor-analytic methods can be used to uncover shared dimensions of cognitive and brain changes, both in mid- and later-life samples. I'll end by showcasing a new methodology we've developed to investigate shared genetic dimensions underlying brain networks, and show how those dimensions covary with cognitive ability and cognitive ageing.