The invention relates to age-related brain diseases, such as Parkinson’s or Alzheimer’s disease. Statistical models based on the regression of measurements with age are inadequate to model the progression of such diseases. As a consequence, the inventors worked on a numerical model to determine a temporal progression for such biological phenomenon, the numerical model being a function in a Riemann manifold.rnSuch model enables to obtain a method for determining the temporal progression of a biological phenomenon which can be implemented on computer and provides better results than statistical models based on the regression of measurements. This determining method may be applied for predicting that a subject is at risk of suffering from such disease, diagnosing a disease, identifying a therapeutic or a biomarker and screening compounds useful as a medicine.rn