In a number of insect taxa, male sexual maturity is not always directly attained at adult eclosion. Sexual maturity is often defined behaviorally as mating success, or physiologically as the point when gametes are produced. Here, we show that the sexual maturation process in males is more complex than previously described and that a measure of maturity also needs to include post-mating traits. In many species mating success is tightly linked with size of the accessory gland (AG). The AG is the production site of a complex cocktail of proteins that are transferred to the female along with the sperm, and which impact on important functions for male reproductive success. Using Drosophila melanogaster we conducted comprehensive behavioral assays of several aspects of male reproductive fitness and demonstrated that the AG has to attain full maturity for males to reach full reproductive competence. We tested males from shortly after, and up to 6 days, post adult eclosion, and considered both pre- and post-copulatory traits. Young males are less likely to gain a mating and show significantly less courtship behavior. Young males that managed to obtain a mating however were inferior in many post-copulatory traits like sperm competitiveness and effectiveness in suppressing female remating. We monitored male AG development over the first 6 days after eclosion and found it to significantly increase in size over the entire period.