Telomere length is a biomarker of cell aging that is hypothesized to contribute to women’s greater longevity. Although most previous studies have found no sex difference in telomere length at birth, it is well established that females have longer average telomere length than males during adulthood. Proposed biological mechanisms underlying sex differences in adult telomere length include differences in sex steroid hormones and body size, which emerge during the pubertal transition. The purpose of this study was to examine the total effect of sex on telomere length during early adolescence and to examine estradiol, total testosterone, and body surface area (BSA; a measure of body size) as potential mediators of sex differences in telomere length. Data were from a population-based sample of 126 female and 109 male Hispanic adolescents aged 8 to 14 years from the Early Life Exposures in Mexico to ENvironmental Toxicants (ELEMENT) study. Relative telomere length (T/S ratio) was measured by the quantitative polymerase chain reaction method; sex steroid hormones were measured using an automated chemiluminescent immunoassay, and BSA was calculated using measured height and weight. Adjusting for age and pubertal status, we found that girls had significantly longer telomeres than boys (β = .13; P < .01), but there were no significant indirect effects of sex on telomere length through any of the proposed mediators. We conclude that sex differences in telomere length are evident during early adolescence but are not explained by cross-sectional differences in sex steroid hormones or body size.