With the increasing prevalence of voice-production technology across societies, clear comprehension while listening to synthetic speech is an obvious goal. Common human factors influences include the listener‟s language familiarity and age. Production factors include the speaking rate and clarity. This study investigated the speaking comprehension performance of younger and older adults who learned English as their first or second language. Presentations varied by the rate of delivery in words per minute (wpm) and in two forms, synthetic or natural speech. The results showed that younger adults had significantly higher comprehension performance than older adults. English as First Language (EFL) participants performed better than English as Second Language (ESL) participants for both younger and older adults, although the performance gap for the older adults was significantly larger than for younger adults. Younger adults performed significantly better than older adults at the slow speech rate (127 wpm), but surprisingly at the medium speech rate (188 wpm), both age groups performed similarly. Both young and older participants had better comprehension when listening to synthetic speech than natural speech. Both theoretical and design implications are provided from these findings. A cognitive diagnostic tool is proposed as a recommendation for future research.