An observational analysis of 1,350 CARDIA participants (mean age ~60, 58% female, 45% Black) published in The Lancet (Jiang et al., 2026) links elevated plasma p-tau217 and amyloid-beta ratios to modest deficits in processing speed and executive function at baseline, with 2.5- to 4-fold higher odds of accelerated decline over five years. This cohort study, not an RCT, carries inherent confounding risks yet benefits from the CARDIA design’s rigorous serial cognitive testing and diverse sampling. Unlike prior work focused on symptomatic cohorts, the findings extend the preclinical window into midlife, aligning with Yaffe’s earlier Lancet Commission updates on modifiable risks that could avert up to 40% of cases. Two additional peer-reviewed sources deepen the picture: a 2023 Neurology meta-analysis (n>8,000) confirming blood p-tau’s superior specificity over CSF for early tauopathy, and a 2024 JAMA Neurology report on ARIC participants showing similar executive-function correlations with plasma biomarkers independent of APOE status. Limitations remain—false-positive rates, restriction to Alzheimer’s pathology (60-70% of dementias), and absence of longitudinal imaging validation—yet the data suggest routine blood panels could soon stratify risk for lifestyle intervention trials decades earlier than current practice.