Plasma biomarkers represent promising tools for the screening and diagnosis of patients with neurodegenerative conditions. However, it is crucial to account for the effects of aging on biomarker profiles, especially in the oldest segments of the population. Additionally, biomarkers in this sample can offer in vivo insights into the physiological mechanisms underlying brain aging while concomitantly supporting cognitive preservation. In this study we analyzed plasma Alzheimer's disease (AD) core biomarkers, neurofilament light chain (NfL), and glial fibrillary acid protein (GFAP) using the Single Molecule Array (SIMOA) platform in 75 cognitively preserved nonagenarians, and compared with baseline samples of 153 volunteers who were cognitively unimpaired (CU) during six years (classified in ≤ 70, and 71 to 85 years of age), and with 108 AD patients. Nonagenarians almost lack the APOEε4 allele, and had significantly higher Aß40, Aß42, p-tau181, NfL, and GFAP, along with a significantly lower Aß42/40 ratio (P&;lt0.001) compared with the two CU groups. NfL and GFAP tripled concentrations in nonagenarians. No differences were noted in any plasma biomarker between the younger and older CU groups. Biomarkers correlated strongly with age only when analyzing together CU controls and nonagenarians. Compared with AD cases, nonagenarians showed lower p-tau181 (P=0.001), higher total tau (P=0.02), and much higher Aß40, Aß42 and NfL levels (P&;lt0.001). The levels of GFAP in nonagenarians were similar to those observed in AD patients. In conclusion, cognitively preserved nonagenarians do not develop the AD biomarker signature and exhibit higher levels of Aß42. However, their threefold increase in NfL and GFAP supports their aging brains are somehow resilient to neurodegeneration. These data support caution in the prognosis of clinical dementia based on NfL and GFAP values. Overall, plasma biomarkers in CU individuals remained quite stable till the eighties.