Older adults exhibit degraded speech comprehension in complex sound environments, which may be related to overall age-related declines in low-level sound segregation. This hypothesis was tested by measuring event-related potentials (ERPs) while listeners identified two different vowels presented simultaneously. Older adults were less accurate than young adults at identifying the two vowels, although both groups improved similarly with increasing fundamental frequency differences (Deltaf0) between vowels. Reaction time data showed that older adults took more time to process stimuli, especially those with smaller Deltaf0. A negative ERP wave indexing the automatic registration of Deltaf0 (the object-related negativity) was reduced in older adults. In contrast, young and older adults showed a similar pattern of neural activity indexing attentive processing of Deltaf0. The results suggest that aging affects the ability to automatically segregate speech sounds.