I develop a life cycle model to evaluate the relative importance of several long-term drivers of real interest rates. The model is a generalization of Gertler (1999), including credit market imperfections and heterogeneity among workers to fully capture the effects of the demographic transition. I calibrate the model to the Brazilian economy, feeding it with profiles for government spending, public debt, productivity growth and demographic variables. The model explains 71 percent of the overall change in real interest rates in Brazil between 2000 and 2019. Demographic factors, especially increases in life expectancy, are the key drivers of the fall in real interest rates. These forces are partially compensated by increases in public debt and social security spending. Moreover, the framework suggests that real interest rates will keep falling over the next 20, reaching a level of 1.5 percent despite reasonable increases in government debt. However, possible effects of risk premia and global rates on domestic interest rates are absent from the analysis.