To examine changes in semantic memory organization and use during aging, we recorded event-related potentials as younger and older adults listened to sentences ending with the expected word, an unexpected word from the same semantic category, or an unexpected word from a different category. Half of the contexts were highly constraining. In both groups, expected words elicited less negativity 300–500 ms (N400) than unexpected ones, and unexpected words elicited smaller N400s when these were categorically related. Whereas younger adults showed the greatest N400 reduction to unexpected but related words in high constraint contexts, older adults showed the opposite tendency. Thus, unlike younger adults, older adults as a group do not seem to be using context predictively. Older adults with higher verbal fluency and larger vocabularies, however, showed the younger response pattern, suggesting resource availability may offset certain age-related changes.