According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.
Randomized Controlled Trial (RCT) with long-term follow-up
Whether daily multivitamin-mineral supplementation causally slows cognitive decline over time in older adults, and whether the effect size can be meaningfully translated into years of cognitive aging. Prospective cohort study with repeated cognitive assessments and biomarker validation
Whether long-term multivitamin use correlates with slower cognitive decline and whether this association aligns with the two-year aging reduction claim using validated normative models. Secondary analysis of existing RCTs with harmonized cognitive data
Whether the pooled effect size from multiple RCTs can be consistently translated into a two-year cognitive aging reduction using standardized, pre-registered modeling methods. Mechanistic sub-study with neuroimaging and biomarkers
Whether the cognitive benefit from multivitamins is biologically plausible and corresponds to structural or functional brain changes consistent with slowed aging. Observational study with instrumental variable analysis
Whether the association between multivitamin use and cognitive aging is causal, not confounded by lifestyle factors.