The distribution of age-of-air in a two-dimensional, zonally averaged model of the atmosphere is examined. The age-of-air is determined from mixing ratios of a fictitious tracer with a linearly increasing tropospheric source. The impact of horizontal mixing is isolated by performing parallel calculations with and without horizontal mixing terms in the tracer evolution equation. We examine the evolution of the tracer over several model years, using different tropical heat sources as well as different amounts of planetary wave mixing and drag. Typical ages in the midstratosphere when horizontal mixing is included range from 3 to over 5 years, depending primarily on the strength of the model's prescribed tropical heat source. These values of age are within the range of observational estimates based on long-lived tracers with increasing tropospheric mixing ratios, such as HF, CO2 and SF6. Age determined from the horizontally unmixed tracer is reduced by 1 to 2 years. However, the evolution of the unmixed tracer is consistent with observed behavior of tropical water vapor oscillations in data from the HAlogen Occultation Experiment (HALOE). We found stronger than expected sensitivity of middle atmospheric age to prescribed heating in the tropical troposphere. This sensitivity in our model arises from small resulting changes to the residual vertical velocity in the tropics. These vertical velocity changes are in turn associated with net heating changes of 0.05--0.10 K d-1 in the tropical lower stratosphere. We argue that changes of this magnitude are within the uncertainties imposed by natural variations in the tropics. ¿ 1998 American Geophysical Union