We developed a near-peer mentoring model for high school youth to mentor middle school youth on how to program using MIT App Inventor. The purpose of this study was to investigate: (a) the effectiveness of the near-peer mentoring model for the mentees; and (b) how the mentees' vicarious experience with the near-peer mentors led to changes in their self-efficacy in computer programming. Findings provide evidence for the effectiveness of the model in improving self-efficacy. A moderation analysis showed how mentor role modeling and perceived similarity contributed to the changes in self-efficacy. Explanations to the findings and their implications are discussed.