A systematic review finds variable MRI derived brain age differences after adult traumatic brain injury and limited evidence on longitudinal change and cognitive associations

Brain age derived from magnetic resonance imaging (MRI) may reflect structural deviation after traumatic brain injury (TBI), although the magnitude, longitudinal course, and clinical implications of this deviation remain uncertain. PubMed, Scopus, and Web of Science were searched from inception to 16 July 2026, supplemented by extensive hand-searching completed on 17 July 2026. Two independent reviewers screened records, extracted data, linked overlapping cohorts, and assessed risk of bias using design-specific Joanna Briggs Institute tools; QUIPS was used only for prognostic-factor studies. Data were categorized according to cross-sectional case-control differences, longitudinal change, and cognitive or clinical associations. A structured narrative synthesis informed by SWiM described findings in their original reported units, with explicit attention to cohort overlap, study design, precision, and risk of bias. Of 338 database records, 162 duplicates were removed and 20 full texts were assessed. No additional eligible full-text report was found through hand-searching. Fifteen reports representing 13 provisional study families were included, and every report contributed to at least one narrative domain. Four T1-based case-control reports provided raw group summaries; however, their populations, models, age-bias handling, confounder adjustment, and variance structures differed. Other evidence included adjusted T1 contrasts, diffusion and regional models, symptom-defined samples, repeated imaging, and cognitive or biomarker associations. Stratification by age-bias correction, injury severity, and post-injury epoch did not reveal a uniform pattern. Across these domains, some reports showed older-appearing brains after TBI, while null or weak findings were also reported; cohort overlap and incompatible reporting limited cross-study inference. The key contribution of the current review lies in defining the recurring obstacles to interpretable brain-age evidence in TBI: inconsistent age-bias correction, clinical heterogeneity of samples and post-injury epochs, overlapping cohorts, and inadequate reporting of outcomes and precision. Available findings indicate structural deviation from normative aging in some TBI populations but do not establish a common effect magnitude or accelerated longitudinal aging. Harmonized reporting, independent validation, and appropriately controlled longitudinal designs are prerequisites for clinical or prognostic interpretation. Not applicable. CRD420261451858.