Studying the process of ageing in humans is dependent on methods for quantitative measurement of the physiological differences between younger and older individuals. Although in many cases the chronological age is used as a measurement of ageing, it provides merely a rough approximation of the actual physiological age of a person. Here, we propose a computer method for measuring the physiological age of the human knee by using quantitative analysis of the radiographic Haralick bone texture. Experiments using longitudinal data from the Baltimore Longitudinal Study of Aging show that the radiographic Haralick texture correlates with the chronological age, and that the ageing of the knee as reflected by the tibial anisotropy and measured using the radiographic texture is not linear to the chronological age. The analysis reveals that the bone ages until the person gets to the age of ,40, then stabilises until the age of around 65, from where it continues to age. A period of rapid ageing was also detected around the age of 55. This shows that periods of rapid morphological changes of the knee occur not only during maturation, but also during ageing.