Background Many clients in contact with social services and (mental) health care have mild to borderline intellectual disabilities (MBID). Yet, administering a full intelligence test may not be feasible. Method In 318 adults and 305 juveniles, the scores on 14 questions that comprise the screener for intelligence and learning disabilities (SCIL) were analysed in relation to the IQs of the participants. Results The SCIL score had good predictive validity for detecting MBID in adults (AUC = 0.93). A cut‐off score of 19 or lower is recommended to detect (a suspicion of) MBID. Test–retest reliability of the SCIL was 0.92. In juveniles, AUC values were 0.91 for 16‐ to 17‐year‐olds, 0.90 for 14‐ to 15‐year‐olds and 0.83 for 12‐ to 13‐year‐olds. Conclusions The SCIL appears to be a time‐efficient tool for screening for MBID in adults (18+) and juveniles of 14 years or older.