Reflective-thinking skills are important in problem-based learning environments as they help learners become deeply engaged in learning. The literature suggests several instructional-design factors (e.g., environment, teaching methods, scaffolding tools) that may prompt reflection in learners. However, it is unclear whether these factors differ based on age or developmental stage. The results of this study indicate that middle-school students perceive the "learning environment" factor as more important to prompting their thinking, while college students perceive the "scaffolding methods" factor as more important. While the elements clustered into two factors, most college students disagreed with their helpfulness in prompting reflective thinking, a finding opposite to that obtained for middle-school students. Different patterns were also found between learners' perceptions of the most helpful elements within each factor. Based on these results, suggestions are given for designing developmentally and age-appropriate PBL learning environments that support reflective thinking.