Sustained attention is the focusing attention process on the selected stimulus for a long time, which can be measured with sustained attention to response task (SART). Our research group aimed to investigate the age-related changes in inhibitory functions with electroencephalography (EEG) in theta oscillations. The EEG of 19 healthy individuals (young, 18-25 years old, M = 21.71, SD = 2.06; elderly 55-70 years old, M = 62.43 , SD = 6.33) was recorded at the F3, Fz, F4, C3, Cz, C4, P3, Pz, P4, O1, Oz, and O2 locations during the auditory SART. The maximum power of the target stimuli was calculated for time-frequency decomposition with a Morlet wavelet in the theta band (4–7 Hz) with three cycles for new analytical methods such as event-related spectral perturbation (ERSP) and intertrial phase-locking (ITC). Results showed that elderly individuals have significantly higher ITC than the younger participants F(1, 12) = 15.63, p < .01 and this increase is seen in block 2 F(1, 12) = 7.52, p < .05 and block 3 F(1, 12) = 10.79, p < .01. Literature shows that the strong frontal midline theta activity which is known to be parallel to executive control and sustained attention. This study showed increased ITC power in elderly individuals compared to younger individuals. This could be interpreted as trial-to-trial fluctuations in the theta band especially during response inhibition in elderly individuals. Furthermore, decreased theta ITC towards the end of the experiment in young participants might be due to mental fatigue.