Telomeres are protective caps at the end of chromosomes, and their length is positively correlated with individual health and lifespan across taxa. Longitudinal studies have provided mixed results regarding the within-individual repeatability of telomere length. While some studies suggest telomere length to be highly dynamic and sensitive to resource-demanding or stressful conditions, others suggest that between-individual differences are mostly present from birth and relatively little affected by the later environment. This dichotomy could arise from differences between species, but also from methodological issues. In our study, we used the highly reliable Terminal Restriction Fragment analysis method to measure telomeres over a 10-year period in adults of a long-lived seabird, the common tern (Sterna hirundo). Telomeres shortened with age within individuals. The individual repeatability of age-dependent telomere length was high (> 0.53), and independent of the measurement interval (i.e. 1 vs. 6 years). A small (R² = 0.01), but significant part of the between-individual variation in telomere length was, however, explained by the number of fledglings produced in the previous year, while reproduction in years prior to the previous year had no effect. We confirmed that age-dependent telomere length predicted an individual’s remaining lifespan. Overall, our study suggests that the majority of between-individual variation in adult telomere length is consistent across adult life, and that a smaller part of the variation can be explained by dynamic factors, such as reproduction.