As the brain ages, oscillatory changes disrupt neuronal communication, contributing to cognitive decline in key areas such as parts of the attention network system. This study explores the effects of multi-session low-intensity transcranial Alternating Current Stimulation (tACS) on the efficiency of the alerting, orienting, and executive control networks in older adults. Using a 16-session theta-gamma tACS protocol targeting the prefrontal cortex, we examined its impact on Attention Network Task (ANT) performance of 76 participants aged 55 to 84 in a randomized, double-blind, sham-controlled design. To account for the influence of brain state, both active and sham tACS groups underwent cognitive n-back training during stimulation. Despite no significant modulations in attention network efficiencies, generalized linear mixed-effect modeling revealed that active tACS negatively influenced overall reaction time (RT) improvements, resulting in poorer ANT performance compared to the sham group. Additionally, active tACS disrupted network correlations post-intervention, particularly affecting the alerting network's interactions with the orienting and executive networks. These findings provide further evidence for the involvement of theta-gamma coupling in attention processes, though without network-specific effects. The results underscore the potential of frequency-specific neurostimulation to modulate cognitive functions but also emphasize the need for caution, as such interventions may inadvertently impair brain network dynamics.