We confirmed strong associations between age and Alzheimer’s disease biomarker concentrations, apart from Aβ42/40: GFAP, r = 0.612 (0.513, 0.695), P < 0.0001; NfL, r = 0.523 (0.409, 0.62), P < 0.0001; P-tau217, r = 0.379 (0.248, 0.496), P < 0.0001; Aβ42/40, r = 0.107 (−0.036, 0.248), P = 0.147. Here, GFAP is linked to early-onset autosomal dominant Alzheimer disease.