The limited capacity of working memory constrains how well we can think and act. Focused attention alleviates this limitation by prioritizing the most relevant mental content at a given time. Retro-cues tap into this ability by guiding attention to one working memory content, thereby improving memory speed and accuracy. So far, few attempts have been made to understand the retro-cue effect through well-established computational models, nor how their parameters track age-related changes and individual differences in focusing efficiency. The present study aims to close these gaps. We applied the drift-diffusion model to the data from a large sample of younger and older adults (total N = 346) that completed four retro-cue tasks. Diffusion modeling showed that retro-cues increased the quality of evidence accumulation, reduced the time taken for retrieval, and changed response conservativeness. Younger and older adults benefited from retro-cues in all tasks and parameters. Yet, age-related decline was observed in the retro-cue boost for evidence quality. Likewise, evidence quality was the only parameter capturing individual differences in focusing efficiency. Our results suggest that people differ in how well they can strengthen and protect a focused representation to boost evidence-quality accumulation, and this ability declines with aging.