This study was performed to determine age- and sex-related differences in lip thickness. Lateral cephalometric images of 220 healthy individuals were taken and the thicknesses of the upper and lower lips were measured. The measurements were performed in three different age groups. Our results indicate that the lower lip thickness, as well as the distance between the most anterior point of contact between the upper and lower lips and the most protruding point of the upper incisor teeth, differed between the two sexes. The lips of males were thicker than those of females. Receiver operating curve analysis was performed to determine the cutoff values to differentiate thickness between males and females. Our results showed that knowledge of upper and lower lips thickness in relation to age and sex may be beneficial to forensic anthropologists, to plastic and reconstructive surgeons, and to orthodontists for more detailed examination, effective treatment, and optimised outcomes.