The immediately-prestimulus electroencephalographic (EEG) brain state influences subsequent event-related processing, dynamically impacting event-related potential (ERP) and behavioural outcomes. Both EEG and ERPs are known to undergo age-related change, yet few have investigated the consistency in their dynamic interrelations in the context of ageing. The present investigation assessed the impact of prestimulus alpha and beta brain states in 20 young (18–26 years) and 20 gender-matched healthy older (59–75 years) adults who completed an equiprobable auditory Go/NoGo paradigm. Prestimulus alpha and beta band amplitudes in their prominent band topographies were separately used to derive Go and NoGo ERPs at 10 ascending levels of prestimulus activity, and ERP components were derived for these levels using Principal Components Analysis. Prestimulus alpha directly modulated Go/NoGo P3a amplitudes across the groups, while beta inversely modulated the young (cf. older) adult NoGo N1–1, each supporting and extending limited prior research. Several novel effects were also uncovered, most notably an inverse relationship between prestimulus alpha and reaction time. Prestimulus alpha and beta were confirmed as significant determinants of the processing outcomes in this task, and the complex pattern of results provides a normative map in healthy ageing.