Mentoring programs that provide guidance and support for disadvantaged youth have expanded rapidly during the past decade in the United States. Research suggests that students with teenage mentors exhibit positive youth development, including enhanced academic self-esteem and connectedness. By contrast, some studies showed that programs that offer teenagers opportunities to interact with their peers may produce unintended consequences, such as problem behaviors. Little research, explores whether teenage mentors are more or less effective than mentors in other age ranges. Using data from the Department of Education's Student Mentoring Program, a randomized evaluation of school-based mentoring programs in the US between 2005 and 2007, Hwang investigate (1) the effects of mentor age on student developmental outcomes; (2) whether or not teenage mentors are more or less effective relative to other age ranges; and (3) the links between mentor age and three crucial domains of youth outcomes: academic performance, scholastic efficacy, and behavioral outcomes. Despite some study limitations, important findings for mentoring programs that serve many at-risk youth in the United States showed students who met with mentors are more likely to exhibit positive youth outcomes than students in the control group; the number of teenage mentors has grown in the recent years; and the findings suggest that mentors whose ages are close to those of mentees have more positive impacts on their mentees. Tables are appended.