Body-mass index (BMI) tends to follow a typical trajectory over the life-course of an individual, increasing in early life while decreasing after middle age. To be able to reflect these trends in the OECD Strategic Public Health Planning for Non-Communicable Diseases (SPHeP-NCDs) model, this paper analyses longitudinal BMI data from 22 countries to build a mixed, autoregressive model predicting an individual’s BMI based on their sex, age and previous BMI. The resulting model shows how young people are likely to see an increase in BMI year-on-year, even if they already have overweight or obesity. It also shows that that a healthy weight in childhood does not protect against future overweight, as BMI continues to increase well into adulthood even for children who start off with a healthy weight. The results of this analysis will be incorporated in the OECD SPHeP NCDs model, to better simulate the longer-term impact of interventions, in particular interventions targeting childhood obesity.