Age estimation is widely applied in several clinical and forensic fields. The radiographic evaluation of dental development is one of the most accepted tools for this purpose. Among the different methods proposed, Demirjian's method was the most extensively used and tested in the medical literature revealing that the original standards for the French-Canadian population tends to over-estimate the age of different population groups. The aims of this study were to evaluate the applicability of the Demirjian method in a sample of Italian children with Down syndrome (DS) and to compare the data with age and gender matched healthy subjects (non-DS). A retrospective study was performed on 146 orthopantomograms of DS individuals aged 6.3-16 years. The mean chronological age (CA) and the mean dental age (DA) were calculated. Using Cohen's kappa statistics, the inter- and intra-examiner agreement was reported as good (k = 0.75) and very good (k = 0.86). The differences between CAs and DAs were statistically significant for males and females (Wilcoxon Signed Rank test; p < 0.05). The median overestimation was 0.6 years in males and 0.9 years in females. Demirjian's method is unsuitable for dental age estimation in DS individuals. The same trend in overestimation was found in the control group. Comparing DS and non-DS subjects, the differences between DAs were not statistically significant for both males and females revealing that the dental development process is similar.