Estimated prevalence of A/T(N) positivity across clinical diagnoses. We estimate that between ages 35 to 55 years, there will be 80% A+, 40% T+, and 10% (N)+ for cognitively stable adults with Down syndrome (DS); 80% A+, 60% T+, and 20% (N)+ for mild cognitive impairment (MCI)‐DS; and 80% A+, 80% T+, and 60% (N)+ for the dementia in DS group. A+ = elevated brain amyloid, T+ = tau pathology present, (N) = neurodegeneration present