In this study, we hypothesized that the tendency toward an age estimation bias when judging age based on facial images was driven by relative comparison with one’s own age, similar to situations of face-to-face communication. Using facial images as stimuli, participants were asked to assess the ages of those in the images in relative terms (younger or older than the participants themselves). We examined the relationship between age estimation bias and participants’ age and gender, as well as the type of facial expression in the images (smiling or neutral). This bias was found throughout most gender and age groups, with the exception of the middle-age female group. Moreover, the bias was greater in men than women, and was influenced by both age and type of expression. These results suggest that the main factors responsible for age estimation bias interact in a complex juxtaposition of variables such as sex, age, and expression.