Objectives This study investigated the best predictor to capture age-related changes in passive-sentence production using a constrained sentence-production paradigm and explored the role of working-memory capacity in relation to the task demands of the sentence-production tasks. Methods A total of 60 participants participated in the study ranging in age from 21 to 86. All were administered a syntactic-priming and a sentence-completion task under either canonical or noncanonical word-order conditions. Results Age was significantly and negatively correlated with sentence-production tasks, and the most demanding condition with a noncanonical word order under the syntactic priming paradigm was the best predictor of aging. Working-memory capacity was significantly and positively correlated with all conditions, but the significant correlation remained only for the most demanding condition (the priming task with a noncanonical word order) after controlling for age. Discussion Sentence-production abilities were vulnerable to aging, and these effects manifested most clearly when the task demands were high enough to tax individuals' cognitive capacity. Working-memory capacity partially accounted for age-related changes in sentence-production abilities.