Telomeres are often used in studies of adults as biomarkers of cellular aging. However, we know little about how telomeres change over time, particularly over the course of the important developmental period of adolescence. We use data on telomere length collected at two points in time spanning adolescence (Year 9 and 15) from the Fragile Families and Child Wellbeing Study to examine cross-sectional and longitudinal patterns in telomere length and adolescent development.