Symptoms of anxiety and depression are commonly comorbid and partially share a genetic etiology. Mean levels of anxiety and depression increase over the transition to adolescence, particularly in girls, suggesting a possible role of pubertal development in the activation of underlying genetic risks. The current study examined how genetic and environmental influences on anxiety and depression differed by chronological age and pubertal status. We analyzed composite scores from child self-reports and parent informant-reports of internalizing symptomology in a racially and socioeconomically diverse sample of 1,913 individual twins from 1,006 pairs (ages 8--20 years) from the Texas Twin Project. Biometric models tested age and pubertal status as moderators of genetic and environmental influences shared between and specific to anxiety and depression to determine whether etiology of internalizing symptomology differs across development as a function of age or puberty. Genetic influences did not increase as a function of age or puberty, but instead shared environmental effects decreased with age. In an exploratory model that considered the moderators simultaneously, developmental differences in etiology were reflected in genetic and environmental effects unique to depression. Results suggest that genetic variance in internalizing problems is relatively constant during adolescence, with environmental influences more varied across development.