In the present study, we investigated effects of phasic alerting on visual attention in younger and older adults. We modelled parameters of visual attention based on the computational Theory of Visual Attention (TVA) and measured event-related lateralizations (ERLs) in a partial report task, in which half of the displays were preceded by an auditory warning cue. Younger adults showed an alertness-related visual processing facilitation: TVA parameter sensory effectiveness a, a measure of visual processing capacity, was significantly increased, and latencies of visual ERLs were significantly reduced following the warning cue. By contrast, older adults did not benefit from the alerting cue: TVA parameter sensory effectiveness a and ERL latencies did not differ between conditions with and without cues. The findings indicate age-related changes in the brain network underlying alertness and attention, which governs the responsiveness to external cues and is critical for general cognitive functioning in aging.