Age information

as an important personal trait

has great potential in safety surveillance

human-computer interaction

multimedia applications

and face recognition.As an emerging biometric information identification technology

face-image based age estimation has gained great attention resently and has become one of the important research topics in machine learning and computer vision.In this paper

we survey most existing commonly used methods in face-image based age estimation

especially focusing on the extraction of age features and classification.Then

we also introduce some face aging databases and evaluation protocols

which are widely used at present.Based on these databases and evaluation methods

a comparison of the performances of several age estimation systems is presented.Finally

the challenges and promising directions of age estimation techniques are discussed.