The 25-item Strengths and Difficulties Questionnaire (SDQ) is a popular screening instrument that assesses childhood psychopathology and prosocial behaviour. While the value of acquiring multi-informant perspectives in the assessment of childhood psychopathology is recognised, the use of the self-report version of the questionnaire is recommended only for children aged 11 years and older. This study aimed to evaluate the psychometric comparability of self-report on the SDQ by children aged 9–10 years relative to children aged 11 years using item response modelling. Item response models were used in a community sample of 7959 children to investigate the structure, item characteristics, and age differences related to self-report by children aged 9–10 years (n=6004), relative to children aged 11 years (n=1955), on the SDQ. Internalizing, externalizing, and prosocial factors explained the covariance of the questionnaire items. There were statistically significant age-related differences on only two items (the “worries” and “distractible” items), but these differences were small in magnitude and did not affect the precision of measurement of the SDQ. Self-report by children aged 9 and 10 years on the SDQ is psychometrically comparable to reports by children aged 11 years after controlling for differences in latent severity. This work expands the utility of the self-report SDQ to children aged as young as 9 years.