After adjustment for age at diagnosis, sex, and APOE alleles, Aβ42, Aβ40, Aβ42/Aβ40, and t-tau were not significantly associated with AAO in the probable AD group (p > 0.05), but NfL showed a significant association (r = −0.183, p < 0.001), which indicated that the earlier the AAO, the higher the level of NfL in plasma. The gene discussed is NEFL; the disease is Alzheimer disease.