The concept of brain age (BA) describes an integrative imaging marker of brain health, often suggested to reflect ageing processes. However, the degree to which cross-sectional MRI features, including BA, reflect past, ongoing and future brain changes across different tissue types from macro-to microstructure remains controversial. Multimodal imaging data of39, 325UK Biobank participants, aged44−82years at baseline and2, 520follow-ups within1.12−6.90years, provide insufficient evidence that BA reflects the rate of brain ageing.