Age at first reproduction is an extremely important life-history trait. Several factors such as nutritional state and age-specific fecundity have been shown to influence time to sexual maturity; however, little work has been done in insects. We addressed this in a stalk-eyed fly (Cyrtodiopsis dalmanni), by testing the hypothesis that time to sexual maturity is associated with the development of male internal reproductive structures. We found that sexual maturity was attained after an increased rate of growth in the accessory glands, several days after mature sperm bundles, and motile sperm were observed in the testes. Although testis development is essential, the results suggest that accessory gland growth is more closely associated with the time taken to reach sexual maturity than is testis growth. When we manipulated the growth of testes and accessory glands via a dietary manipulation, we found that delayed growth rates increased the time taken to reach sexual maturity. Among the delayed individuals, sexually mature males had larger accessory glands, but not testes, than did immature males. In adult males, mating frequency was significantly positively correlated with accessory gland size, but not with testis length or body size. We conclude that accessory gland size is a critical determinant of sexual maturity and male mating frequency in this species.