The shifting landscape of brain-wide myelination across the human lifespan.

Despite the central role of myelination in brain function and plasticity, its characterization remains fragmented, with existing studies typically constrained either spatially, by focusing on selected brain structures, or temporally, by covering only limited age ranges. Here, we provide a comprehensive brain-wide map of myelination across the lifespan, leveraging T1w/T2w ratios from 214 brain regions in individuals aged 0-100 years. This fine-grained spatiotemporal mapping reveals regional heterogeneity and timing differences in myelination and demyelination, uncovering fundamental organizational principles that govern myelin dynamics across the lifespan. Our analysis also reveals that changes in T1w/T2w ratio across all brain regions follow a triphasic pattern: rapid infancy growth (primary myelination phase), adolescence and adulthood reacceleration (secondary myelination phase), and late-life decline (demyelination). To complement our analysis, we examined regional brain volume and cortical thickness to determine how macroscale structural changes are related to microscale myelination across the lifespan. Our work provides a comprehensive population-level reference for characterizing normative myelination trajectories and for interpreting alterations associated with myelin-related neurological disorders.