Purpose The aim of this study was to compare the mandibular posterior space in subjects with skeletal class II division 1 and division 2 malocclusions in two different age groups. Methods Pretreatment cephalometric radiographs of 160 patients from 9 to 13 years of age (n & x202f;= 80) and 15 to 18 years of age (n & x202f;= 80) with class II division 1 or division 2 malocclusion were used for the study. Equal numbers of male and female patients (n & x202f;= 20) were included in the two age groups in each of the malocclusion groups. Eight linear and six angular measurements were taken for mandibular posterior space evaluation. The data obtained from the study were compared using the independent t-test. Results In the present study, the mandibular posterior space was greater in both malocclusion groups in subjects aged 15-18 years than in those aged 9-13 years (p & x202f;< 0.05). The available posterior spaces behind the mandibular first molars were 4.4 & x202f;mm and 6.3 & x202f;mm in females (class II divisions 1 and 2, respectively) and 5.3 & x202f;mm and 7 & x202f;mm in males (class II divisions 1 and 2, respectively) in the 9- to 13-year-old age group. This space increased significantly by 6.9 & x202f;mm (p & x202f;< 0.001) and 3.2 & x202f;mm (p & x202f;< 0.01) in females (class II division 1 and 2, respectively) and 3.8 & x202f;mm (p & x202f;< 0.01) and 3 & x202f;mm (p & x202f;< 0.01) in males (class II division 1 and 2, respectively) in the 15- to 18-year-old age group. Conclusion Class II division 1 and division 2 malocclusions had similar and inadequate posterior space dimensions for the eruption of the third and an unknown portion of the second molars.