Studies on the population ecology, behaviour and social structure of ungulates often require accurate measurements of the age of individuals. However, to date there is no simple noncontact method of determining the ages of individuals. The horns of ungulates are structurally complex, and grow and change shape as animals age. If 3D models of the horns of individuals of known ages are built, the ages of other individuals can be estimated from photographs of their horns by comparison with the reference models. This paper describes a new age-determining method for blue sheep (Pseudois nayaur) using horn geometric characteristics and computer vision techniques. The procedure consists of three stages: 1) creating 3D reference images based on the horns of individuals of known age (22 individuals, two horns per individual; age range 1.5 to 15.5 years); 2) creating new 3D images of the horns of target individuals, which are then rotated on a computer using an in-house R script to obtain images at stepwise intervals of 1°; and finally, 3) by calculating the similarity between the new and the reference horn images. A final image with the highest similarity to the reference images is generated, and used to determine the actual age of the target individual. Of the 30 photographs used to evaluate our new method the age of the target individual in every picture was correctly determined. Our new method has potential use for studies of all wild/feral sheep and probably also other ungulate species.