Information on the degree to which individuals can make accurate estimations of someone’s age can be of importance in different legal contexts, such as for example child sexual abuse cases in which the victim is an adolescent and asylum procedures. There is, however, a scarcity of studies concerning age estimations conducted with young target persons. Using facial photos of target persons aged 12–18 years, we investigated the effects of gender, age, and ethnicity of both targets (n = 240) and observers (n = 869) on the accuracy of age estimation. We also investigated the effects of targets’ facial expressions (neutral or smiling), use of makeup, and photo quality. Participants overestimated the age of the adolescents by, on average, 3.51 years. Participants overestimated the age of young adolescent girls to a greater extent than that of younger boys. Men made larger overestimations than women. Participants also estimated smiling targets as being older than targets with neutral facial expression, and the age of girls with makeup to be older than girls without makeup. Because there was considerable variation in the accuracy of estimations, and overestimates were common, we conclude that the ability of individuals to estimate the age of adolescents is generally low. This might have important legal implications.