<p>Alzheimer’s disease (AD) is characterized by the accumulation of amyloid-β (Aβ), yet the specific link between plaque burden and cognitive decline remains a subject of intense investigation. This paper presents a mathematical model that simulates the coupled dynamics of Aβ monomers, soluble oligomers, and fibrillar species in the brain tissue. By modifying existing governing equations to include a dedicated conservation equation for Aβ monomers, the model explores how various microscopic processes, such as primary nucleation, surface-catalyzed secondary nucleation, fibril elongation, and fragmentation, contribute to macroscopic disease progression. Central to this study is the concept of “accumulated neurotoxicity” as a surrogate marker of biological age, defined as the time-integrated concentration of soluble Aβ oligomers. Unlike plaque burden, accumulated neurotoxicity cannot be reversed, and the harm it causes depends critically on the sequence of events that produced it. Numerical results demonstrate that while plaque burden and neurotoxicity both increase over time, their relationship is non-linear and highly sensitive to the efficiency of protein degradation machinery. Specifically, impaired degradation causes biological age, defined in terms of accumulated neurotoxicity, to advance considerably faster than calendar age. The model further identifies oligomer dissociation and fibril fragmentation as potential protective mechanisms that can counterintuitively reduce neurotoxic burden by diverting monomers away from the soluble oligomer pool. These findings provide a quantitative basis for understanding why individuals with similar plaque burdens may experience vastly different cognitive outcomes, underscoring the importance of targeting soluble oligomers early in therapeutic interventions.</p> Graphical Abstract <p></p>