This paper considers the age of air in a ventilated space with multiple supply inlets; the local mean age (LMA) of air is different from one inlet to another in multiple supply inlet situations. The purpose of this paper is to investigate theoretically the relations between the LMAs arising from each supply inlet and the overall combined LMA for all inlets. Transient concentration distributions are calculated using a CFD package in a simple room configuration with two inlets and one outlet, to verify the relations. Tracer gases are injected with a step-up method at each supply inlet individually, and at both inlets simultaneously. As a result, it can be found that the steady state concentration, with a continuous tracer injection at each supply inlet, works as a weighting factor for the corresponding LMA in calculating the overall combined LMA for the multiple inlet situations.