The mean age of all the particles inside the control volume at time t is the first moment of the age distribution:
The mean residence time or mean transit time, that is the mean age of all the particles leaving the control volume at time t, is the first moment of the residence time distribution:
The mean age and the mean transit time generally have different values, even in stationary conditions:
* : examples include water in a lake with the inlet and outlet on opposite sides and radioactive material introduced high in the stratosphere by a nuclear bomb test and filtering down to the troposphere.
* : E and I are exponential distributions. Examples include radioactive decay and first order chemical reactions (where the reaction rate is proportional to the amount of reactant).
* : most of the particles entering the control volume pass through quickly, but most of the particles contained in the control volume pass through slowly. Examples include water in a lake with the inlet and outlet that are close together and water vapor rising from the ocean surface, which for the most part returns quickly to the ocean, while for the rest is retained in the atmosphere and returns much later in the form of rain.