This paper presents the results from the experimental and numerical study of a room with mixing ventilation, focused on the local mean age of air (LMA). The measurements were performed using the tracer gas concentration decay method. The numerical predictions were obtained from the computational fluid dynamics (CFD) module of the latest version of the ESP-r software. In order to address the requirement for a proper choice of the number of control volumes in CFD, the analysis is made by comparing the results from two consequently finer numerical grids. Some guidelines for practising engineers are given concerning the number of numerical grid points and their distribution, reasonable for buildings energy simulations. The guidelines are based on common CFD rules supported by examples from the presented computations. Both the fine and the coarse grid computations gave results, which were very close to the measurements. An important issue of the present study is that the coarse grid computations are reasonably close to the measured values, which allows the computation of LMA to be made on relatively coarse numerical grids sometimes (as in the present study) as low as approximately 1000 grid points.