High frequency loss is common among the elderly. It is usually accompanied with other hearing deficits, such as understanding speech in noise. In this study we explored the ability of the elderly compared to young adults to detect mistuned harmony. We have used data from different studies which revealed degradation in detecting mistuned harmony as a function of age, focused at higher frequencies. This study examines whether the degradation of the elderly hearing ability can be explained exclusively by cochlear changes, caused by outer hair cells (OHC) loss, or other, cognitive abilities should be taken into account. The study's methodology was based on a decision making model, It comprises of a time domain cochlear model which assumes older age means less active OHC, A stochastic neural model that represents the auditory nerve firing pattern, and an optimal decision rule. The model predictions were compared to the experimental data. OHC loss caused degradation caused a decrease in the ability to detect the mistuned harmony, especially at high frequencies. We thus conclude that most of the degradation ability of the elderly in the task of mistuned harmony was caused by OHC loss. That's while many researchers tended to attribute it to cerebral processing.