How to track the lifetime of a particle
 

Hi everyone. :)
The case is simple . One inlet, One outlet and some walls. All of the walls are set as stick/trap.
Openfoam uses a particle property named ' active' to decide whether a particle is trapped to a wall and is therefore not considered.
There is another properties named 'age' to tell us the time passed since the particle was injected.
However, it seems that 'age' keeps growing when the particle is already trapped to the walls. I cannot find a way to the lifetime of the trapped particle since the injection time of specific particle is not recorded.
I guessed the injection time can be calculated by 'orginId' but it failed to give me the right answer. (The number of the particles/parcels during a time step injected is fixed)