A new method is developed to characterise impeller mixing performance of a stirred tank reactor with mean age distribution. The spatial distribution of mean age reveals the non-uniform distribution of local blend time. The mean age frequency function is found to scale with impeller speed and the ratio of the impeller diameter to the tank diameter. A blend time is defined from the mean age frequency function and this blend time is found to agree very well with the blend time correlations in literature. This method can be used to predict blend time in a stirred tank reactor using CFD with only a small fraction of the CPU time required by the current method of solving the time dependent tracer scalar equation. With this method, CFD can be efficiently used for practical reactor analysis, design, and scale-up.