Objective Due to increasing numbers of adult patients presenting to orthodontic practices, an increase in incidental findings on diagnostic X‑rays, which are the cornerstone of orthodontic diagnostics, is expected. This raises the clinically relevant question of whether an age effect exists regarding prevalence, localisation and severity of incidental findings on orthodontic diagnostic X‑rays. Materials and methods The clinical, primarily retrospective study examined pathological incidental findings from 600 orthopantomograms (OPT) and lateral cephalogram (LC) images in two groups of orthodontic patients (group I: 150 children/adolescents, age 11.89 ± 2.47 years; group II: 150 adults, age 27.03 ± 10.42 years). Prevalence, localisation and severity of the findings were recorded based on a classification sheet. The assessment was done by three experienced examiners following a systematic approach along the nine locations: mandible, maxilla, dentition, paranasal sinuses, temporomandibular joint, cranial base, orbit, cervical spine, soft tissues. Results In all, 1458 incidental findings were detected, with 66% of the findings having occurred away from the dentition. There was a significant age effect (p < 0.001) with respect to the prevalence of incidental findings (group II—adults 1026 findings—OPT: 566/LC 460 vs. group I—children/adolescents 432 findings—OPT: 221/LC 211). Regarding localisation, incidental findings in adults commonly occurred in the dentition, paranasal sinuses and mandibular regions. Furthermore, analysis of the LC images revealed significantly more incidental findings in the area of the cranial base and cervical spine in adults (p < 0.001, p = 0.003). Categorisation according to the severity of the incidental findings showed that 33% of the incidental findings needed further diagnostic investigation and possibly treatment by other specialities. Conclusion Diagnostic assessment using orthodontic diagnostic X‑rays results in a high prevalence of incidental findings away from the dentition. Particularly in adults, a large number of incidental findings outside the dental/alveolar region may be expected on orthodontic diagnostic X‑rays. Thus, a structured approach during diagnostic assessment is required to minimise the extent to which incidental findings of clinical relevance are overlooked.