Long term forecasting of mobility is a major challenge for urban transport planning. The demographic approach based on data from periodic surveys provides insight into the behavioural dynamics of individuals belonging to different cohorts at various stages of their life cycle. The decomposition of temporal effects into an age effect and a cohort effect makes it possible to define a standard profile over the life cycle: this is the fundamental concept of the “age-cohort” model for predicting long-term urban mobility. The model is applied to the agglomeration of Lille, where three mobility surveys were conducted at 11-year intervals.