Mean age theory was applied towards predicting just suspended speed in mixing tanks by evaluating multiphase mean age near the bottom surface through strategic zone selection. Multiphase mean age equations were solved only in a thin section along the bottom of the vessel (similar to 1% of the vessel height), allowing the mean age in proximity to the bottom to be computed. A rigorously defined method for open systems and a modified method for closed systems using modified boundaries provided equivalent results. The technique was accurate within 1-3% of experimental values across a range of solid densities, solid fractions, and particle sizes while using multiple impeller types and vessel geometries. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.