Summary1. Age‐related reductions in stand biomass accumulation are frequently observed in old‐growth forests. The phenomenon may be caused by reduced production, increased mortality or both. The relative importance of production and mortality is not well studied, so the mechanisms controlling age‐related decline of stand biomass accumulation remain unclear.2. In this study, conducted in a Quercus‐dominated deciduous forest in the Northeastern USA, we examined whether age‐related decline in stand above‐ground biomass (AGB) accumulation could be explained by reduction of above‐ground net primary production (NPP) (growth of surviving trees) that may be associated with (i) physiological constraints within individual trees or (ii) changes in stand structure, or by (iii) age‐related, increasing tree mortality in stands up to 135 years old. Few previous studies have tested these hypotheses simultaneously within the same forest.3. We did not find evidence for a reduction in individual tree growth associated with age‐related physiological constraints, in terms of foliar carbon assimilation capacity, photosynthesis/respiration balance, nitrogen availability or hydraulic constraints on carbon gain. Over the period of 1937–2006, we did not observe alterations in stand structure, and the above‐ground NPP of the Quercus forest was generally stable.4. However, we did find that the primary mechanism driving age‐related decline of stand AGB accumulation was biomass loss due to the death of large, dominant trees. Our results indicate that shifts in mortality from the loss of small trees to the loss of large trees, rather than changes in above‐ground NPP, drives age‐related decline in stand AGB accumulation in this forest.5. Synthesis. We found that within the range of stand development stages analysed, the age‐related decline of stand AGB accumulation in a Quercus‐dominated forest was primarily due to mortality of large, dominant trees and not due to changes in above‐ground NPP associated with tree physiology, individual tree growth or stand structure. This result indicates that tree demography and the influence of climate change on disturbances may need to be integrated into models to predict the change of above‐ground carbon stock of some old‐growth forests.