The time that a particle of fluid has been in a control volume (e.g. a reservoir) is known as its age. In general, each particle has a different age. The frequency of occurrence of the age  in the set of all the particles that are located inside the control volume at time  is quantified by means of the (internal) age distribution .
At the moment a particle leaves the control volume, its age is the total time that the particle has spent inside the control volume, which is known as its residence time. The frequency of occurrence of the age  in the set of all the particles that are leaving the control volume at time  is quantified by means of the residence time distribution, also known as exit age distribution .
Both distributions are assumed to be positive and to have unitary integral along the age:
In the case of steady flow, the distributions are assumed to be independent of time, that is , which may allow to redefine the distributions as simple functions of the age only.
If the flow is steady (but a generalization to non-steady flow is possible) and is conservative, then the exit age distribution and the internal age distribution can be related one to the other:
Distributions other than  and  can be usually traced back to them. For example, the fraction of particles leaving the control volume at time  with an age greater or equal than  is quantified by means of the washout function , that is the complementary to one of the cumulative exit age distribution: