Age estimation by third molar mineralization has been developed for a period of time. Recent studies showed that impaction status has an influence on the rate of the third molar mineralization in Europids and male Black African populations. In this regard, the present study was conducted to determine whether the impaction status could delay the chronological process of third molar mineralization in northwestern Chinese population too. A total of 3,512 digital orthopantomograms of 1,255 male and 2,257 female northwestern Chinese subjects aged from 11 to 26 years were assessed based on the formation stages described by Demirjian et al. with two modifications. Mineralization stage and impaction status were determined for all third molars. Statistical measures were calculated at stages C-H in the male and female gender for both impacted and non-impacted third molars. It was ascertained that the mean ages with impacted third molars at stages C-H were 0.02-1.42 years higher in males and 0.04-1.52 years higher in females than those with non-impacted third molars, but statistical differences were only found at stages C, D, and E in males and stages D and E in females. The probabilities of being under 14, 16, or 18 years of age with non-impacted third molars were all higher than those with impacted third molars. The results prove that impacted third molars show significant slower mineralization than non-impacted ones at stages D and E in both males and females of northwestern Chinese population. It is recommended to consider the influence of impaction on the rate of third molar mineralization for dental age estimation.