In Alzheimer's disease (AD), the cerebral pathological changes begin many years before the clinical manifestation of the disease. Biomarkers for AD, such as the cerebrospinal fluid (CSF) concentrations of amyloid-β1-42 (Aβ1-42) and tau phosphorylated at threonine 181 (pTau181), may reflect these cerebral changes relatively early. Accordingly, cognitively healthy subjects at risk for AD often have altered CSF concentrations of Aβ1-42 and pTau181. In this study, we assessed the effects and interaction of two strong risk factors for AD, aging and the presence of the APOEε4 allele, on the CSF Aβ1-42 and pTau181 concentrations in 280 adults with normal cognition across the lifespan. For comparison, we further included 152 patients with probable AD. We found significant effects of age on the CSF Aβ1-42 and pTau181, and of the APOEε4 genotype on the Aβ1-42 levels in the cognitively normal participants. Carrying the APOEε4 allele was associated with a significant decrease of the Aβ1-42 concentrations in middle-aged and older participants. In the group of participants with AD, the Aβ1-42 levels were significantly lower in the APOEε4 carriers compared to the non-carriers. These findings demonstrate significant age effects on the CSF Aβ1-42 and pTau181 across lifespan. They also suggest that the decrease of Aβ1-42, but not the increase of pTau181 CSF levels is accelerated by the APOEε4 genotype in middle-aged and older adults with normal cognition.