Successful spoken language comprehension depends upon both sensory and cognitive processing. Since older adults often experience declines in one or both of these domains, and perform worse on some language processing tasks than younger listeners, an important question is how declines to both auditory perception and cognitive abilities affect speech perception. We examined whether "auditory inhibition" (which has previously been linked to lexical competition effects in older listeners), and "response consistency" (RC; an index of encoding fidelity obtained from the Auditory Brainstem Response to the onset of clicks), predict competition in spoken word recognition in younger and older listeners. In two experiments, we used eye tracking to investigate individual differences in lexical competition during spoken word recognition. RC predicted the size of phonologically- and lexically-based competition effects. Specifically, in Experiment 1, RC correlated with competition between onset (cohort) competitors for younger and older participants. In Experiment 2, RC was related to manipulations of word frequency, cohort density, and neighborhood density for older participants. Importantly, auditory inhibition never accounted for listener variation in fixations to target words, which is inconsistent with previous findings in older adults. This indicates that listener variation in the quality of the neural response to sounds in both younger and older listeners is linked to the speed of lexical access. Future studies should 1) measure encoding fidelity in older listeners (e.g., RC) in addition to auditory threshold, and 2) control for low-level sensory processing differences even when research questions are aimed at higher levels of language comprehension. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]