We extend the concepts of the mean age derived from the age spectrum and the age of air derived from the time lag in the middle atmosphere by specifying the tracer source gases uniformly in the lower troposphere. The concepts are illustrated by use of both a time-independent one-dimensional diffusive model and a globally balanced two-dimensional middle atmosphere model. We quantitatively examine several factors that may cause the difference between the mean age and the age of air. It is found that the mean age and the age of air can differ by any amount in general for a time-dependent transport operator. For a time-independent transport operator the age of air derived by integrating a model for a finite time is less than the mean age. The age of air at the stratopause is about 5 to 6 years in our globally balanced two-dimensional middle atmosphere model. In the mesosphere the age of air changes with season significantly due to the reversal of the mesospheric meridional circulation between the summer and the winter. With a chemical lifetime of SF6 above 30 km being ~38 years our two-dimensional model shows no signs of overestimate of age of air for SF6 at 30 km. It is also demonstrated that stratospheric mean age is sensitive to the horizontal mixing in the troposphere. Weak horizontal mixing in the troposphere will reduce the mean age in the upper stratosphere but increase the mean age in the high-latitude lower stratosphere. The former is caused by diminishing the recycling of air parcels between the troposphere and stratosphere, and the latter is caused by minimizing the portion of tracers directly transported upward from the high-latitude troposphere into the stratosphere. ¿ 2000 American Geophysical Union