1. A method of characterizing wrinkles, sign lines, slackening and/or loosening of the skin of the face of a person, comprising the following steps: a) taking at least one digital image of at least one predetermined skin face surface area where said wrinkles, sign lines, slackening and/or loosening of the skin occur, selected from an outer zone of the eye or crow's-foot, and a skin zone of the nasogenian furrow; said image being defined by a multiplicity of pixels with the aid of a digital video camera or a digital picture camera, that is transmitted to a digital image processing device; b) splitting the recorded digital image into three colour planes: red, green, blue, termed R, G, B, with the aid of said image processing device; c) extracting a single of these planes; d) calculating, by appropriate means of calculation, on the basis of this extracted plane, at least the variance of the grey levels over the set of pixels of the image as parameter representative of the homogeneity of the skin, and optionally at least one of the following parameters: i) the mean grey level of the image; ii) the surface area of the outer zone of the eye and skin zone of the nasogenian furrow of the said image; e) recording on a device for recording digital data, the calculated variance of the grey level as a parameter characterizing the wrinkles, sign lines, slackening and/or loosening of the skin of the face; and f) preparing a calibration curve of age versus the variance of the grey levels for a representative group of persons; thereby characterizing said wrinkles, sign lines, slackening and/or loosening of the skin of the face, wherein the calibration curve takes into account the mean of at least the variance obtained from a plurality of images from a plurality of ageing zones of the face from a plurality of individuals.