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Although, there has already been much research on the differences between sexes in fingerprint ridge density and its variability in the Thai population, such studies have not included native northeastern Thais aged between 14 and 24 who are descended from northeastern Thai ancestry. This study intends to determine the topological, age-grouping and sexual differences in fingerprint ridge density (RD) in such populations. Fingerprints were collected from 353 unrelated volunteers (191 males and 162 females) and classified into three groups, that is, group A (total subjects), group B (14–18 years old) and group C (18–24 years old). RD was assessed for two topological areas, radial and ulnar. Significant differences between genders and age groups were obtained in both counting areas. Females exhibit higher RD i.e. narrower ridges, than males. A decrease in RD values with increasing age was also detected. The RD threshold for discrimination of sexes, computed based on Bayes’ theorem, was achieved in all groups and counting areas, enabling its use in forensic investigation.