Hello!
The approach with the momentum source would work , in the sense that the mean age would not be “reset” at the inlets. And technically you could use a power source with a negative power (which acts as a heat sink). You would still have to explicitly define some fresh air inlet, since you can’t have a simulation only with outlets.
In any case, this approach sounds much more complex, and it’s probably not something I would do, unless I am strictly interested in the mean age and nothing else.
Cheers