Evidence from neuropsychological studies of individuals with brain damage post-stroke has supported the separation of working memory (WM) capacities for semantic (word meaning) and phonological (speech sound) information. These separate capacities have been shown to play different roles in supporting multiword language production, with semantic WM being particularly critical for the fluent production of multiword speech. The current study investigated the role that age-related declines in phonological and semantic WM play in language production. Using spontaneous production tasks and eye tracking during constrained sentence production, I investigated the characteristics of healthy young and older adults’ speech and speech planning processes and their relation to WM. These two methods provided converging evidence for the role of semantic WM in planning multiple content words during language production. In older adults’ narrative production, higher semantic WM capacity was associated with more elaborative speech. In sentence production with eye tracking, young adults with greater semantic WM capacity showed a greater tendency to look to a second noun in an initial phrase before speech onset, suggesting that they were planning both nouns before speaking and that this planning required semantic WM. The fact that older adults showed semantic WM effects in narrative production and young adults only showed effects in the eye gaze data highlights the advantage of combining eye gaze data with more standard behavioral measures in investigating the cognitive processes occurring before speech onset.