IntroductionRadiographs of the hand and teeth are frequently used for medical age assessment, as skeletal and dental maturation correlates with chronological age. These methods have been criticized for their lack of precision, and magnetic resonance imaging (MRI) of the knee has been proposed as a more accurate method. The aim of this systematic review is to explore the scientific and statistical evidence for medical age estimation based on skeletal maturation as assessed by MRI of the knee.Materials and methodsA systematic review was conducted that included studies published before April 2021 on living individuals between 8 and 30 years old, with presumptively healthy knees for whom the ossification stages had been evaluated using MRI. The correlation between “mature knee” and chronological age and the risk of misclassifying a child as an adult and vice versa was calculated.ResultsWe found a considerable heterogeneity in the published studies —in terms of study population, MRI protocols, and grading systems used. There is a wide variation in the correlation between maturation stage and chronological age.ConclusionData from published literature is deemed too heterogenous to support the use of MRI of the knee for chronological age determination. Further, it is not possible to assess the sensitivity, specificity, negative predictive value, or positive predictive value for the ability of MRI to determine whether a person is over or under 18 years old.Key Points•There is an insufficient scientific basis for the use of magnetic resonance imaging of the knee in age determination by skeleton.•It is not possible to assess the predictive value of MRI of the knee to determine whether a person is over or under 18 years of age.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00330-023-09546-8.