Older listeners often experience communication difficulties in noisy environments. Augmented dichotic left ear disadvantages (LEDs), demonstrated by substantial left ear word recognition difficulties when competing speech is simultaneously presented to the right ear, have been shown to contribute to age-related speech understanding difficulties. However, age-related peripheral hearing loss confounds the interpretation of interaural asymmetry differences between young and elderly groups. It is of interest to ask whether age-related changes in the LED might be observed in middle-aged persons, where the problem of age-related peripheral hearing loss can be more easily controlled. Therefore, the current study compared interaural asymmetry in an event-related potentials (ERP) task between young and middle-aged listeners. Although augmented LEDs have been consistently demonstrated in elderly listeners during dichotic listening, the degree of LEDs in middle-aged listeners is relatively unknown. Furthermore, it is unclear how LEDs are manifested in real life situations. Accordingly, the current study evaluated the effect of middle age on interaural asymmetry in a quasi-dichotic competing speech paradigm that reflected a more natural listening environment. Auditory ERPs were elicited in response to a word-pair semantic categorization task presented through a front loudspeaker while irrelevant, but semantically meaningful continuous discourse was presented through either right (Competition Right-CR) or left (Competition Left-CL) loudspeakers. Young (n=20) and middle-aged (n=20) females with normal hearing were tested. The N400 component of the auditory ERP showed significantly greater negativity in the CR than the CL condition in middle-aged females when word pairs were unrelated. This indicated that the CR condition placed significantly greater linguistic and cognitive loads on the left ear than the CL condition placed on the right ear. However, the augmented LED was found only in the most difficult semantic judgment task. The LPC component of the auditory ERP showed substantially greater LPC peak amplitude in the frontal regions of the scalp in the middle-aged group than in the young group. This suggested that the middle-age group required additional attentional/cognitive resources from frontal brain areas to compensate for increased processing demands of the semantic processing task. The implications from this study are discussed in relation to research in auditory aging.