Aging cholinergic basal forebrain neurons show a 50‑100% increase in rapid calcium buffering that narrows Ca2+ signals, augments afterhyperpolarization and blocks LTP, yet this rigidity is reversible by caloric restriction 1. In contrast, the same neurons lose calbindin with age, rendering them vulnerable to excitotoxic degeneration in Alzheimer’s disease 3. We hypothesize that the bidirectional outcome hinges on a dynamic phosphatase/kinase switch governed by calcineurin activity versus CaMKII signaling within calcium microdomains near the plasma membrane. When buffered Ca2+ rises, calcineurin dominates, dephosphorylating downstream targets (e.g., actin‑regulating proteins) and stabilizing dendritic spines in a low‑motility state that favors over‑consolidation of existing synaptic weights. Simultaneously, elevated mitochondrial Ca2+ uptake via MCU fuels ATP production that reinforces the AHP, further suppressing spike‑dependent plasticity. Intermittent, low‑dose inhibition of calcineurin (e.g., FK506 at sub‑immunosuppressive concentrations) during brief episodes of environmental novelty would transiently shift the balance toward CaMKII‑mediated phosphorylation, promoting spine turnover and LTP without chronically raising cytosolic Ca2+ to excitotoxic levels. Crucially, this intervention should be ineffective—or harmful—in calbindin‑depleted neurons because their reduced buffering capacity permits larger Ca2+ transients that overwhelm the compensatory kinase response, leading to calcineurin‑independent activation of proteases and tangle formation. To test this, we propose three measurable predictions in aged mice: (1) Acute FK506 administration paired with novel object exposure will increase two‑photon measured spine turnover in basal forebrain cholinergic neurons by >30% relative to vehicle, accompanied by improved reversal learning performance. (2) Chronic FK506 dosing will elevate cytosolic Ca2+ markers and calpain activity selectively in calbindin‑low neurons, detectable via FRET‑based Ca2+ sensors and increased phospho‑tau immunoreactivity. (3) Genetic reduction of MCU expression in cholinergic neurons will mimic the effects of acute calcineurin inhibition, lowering AHP amplitude and rescuing LTP even without pharmacological treatment. Each prediction is falsifiable: failure to observe spine turnover changes with acute FK506+novelty, lack of calbindin‑specific toxicity under chronic dosing, or absence of LTP rescue after MCU knock‑down would refute the switch mechanism. This framework reframes cognitive aging not as a uniform decay but as a tunable balance between stabilizing phosphatases and plasticity‑promoting kinases, where therapeutic success depends on matching the direction of intervention to the neuron‑specific calcium buffering phenotype.