The sensitivity of mean age of air in the stratosphere to the vertical structure of mass exchange between the tropics and midlatitudes is examined with a two-dimensional model with consistent advective and diffusive transport. We use estimates of mean age of air and age spectra in the midlatitude lower stratosphere, derived from observations of CO2, to constrain transport from the tropics to midlatitudes. We show that to reproduce the latitudinal gradient in mean age in the lower stratosphere as well as the bimodal age spectra derived for the midlatitude lower stratosphere, the rate of horizontal transport from the tropics to midlatitudes between 20--30 km must be slower than that at higher and lower altitudes. The relative rates of meridional transport above and below this region determine the separation of the two peaks in the age spectra and the overall mean age of air in the lower stratosphere. Slow transport between 20--30 km can generally not be obtained by reducing only the diffusive component of transport. In the model, gravity wave drag must also be reduced in the lower stratosphere to prevent strong horizontal advective transport across the subtropics. We also show that adjusting the diffusive component of transport independently of the meridional circulation can produce mean ages different from those calculated in the fully coupled model by as much as 2 years. ¿ 2001 American Geophysical Union