The purpose of this study was to investigate whether patient age of 16 years and under is a valid “red flag” for back pain, by determining how often magnetic resonance imaging (MRI) investigations in these patients demonstrated significant pathology. This was a retrospective review of cases over a five-year period (2008–12). The radiology database was interrogated to identify all patients aged 16 and under who had undergone an MRI scan of their lumbar spine for a primary complaint of low back pain. All emergency and inpatient admissions were excluded from the study. Casenotes of each of these patients were analysed for demographics, clinical features, diagnosis and outcome. After exclusions, 98 eligible cases were identified. The age range of these patients was 2–16 years (mean age 12.63 years). The MRI scan found no abnormalities in 71.4% of cases. In the scans with positive findings, there were 8 cases of spondylolysis, 3 spondylolistheses, 9 cases of disc degeneration and 5 cases of Scheuermann's. Tumour or infection were found in only 3% of cases, (2 cases sacroilitis, 1 sacral chondroblastoma); there had been sacral or sacroiliac tenderness in each of these cases. In keeping with other recent studies, this study shows that the diagnostic yield of MRI in patients under the age of 16 with low back pain is relatively high (28.6%). However, scan findings did not significantly alter management in the vast majority of cases (97%). Serious pathology (infection or tumour) was found in only 3% of cases. We therefore suggest that an age of 16 years or less, in isolation, should not be a “red flag” indicator for low back pain. We do however advise a lower threshold for imaging in patients presenting with sacral region pain and tenderness