Correlates of axonal content in healthy adult span: Age, sex, myelin, and metabolic health

•Intra-cellular volume fraction (ICVF), a proxy of axonal content, shows region-specific linear and quadratic associations with age.•ICVF and myelin water fraction were positively correlated in all examined regions.•Men showed greater ICVF than women, which was accounted for by intracranial volume.•Neither metabolic syndrome risk score nor adiposity scores were related to ICVF.•The adiposity- or metabolic syndrome-myelin associations were independent of ICVF.