Due to a trade-off between current and future reproduction, costly reproductive investments should be increased towards the end of a lifespan when the probability of reproduction becomes low (terminal investment hypothesis). We investigated age-related changes in male reproductive investment towards courtship display and the spermatophore in three age classes (young, middle-aged and old) of a monandrous moth, Ostrinia scapulalis. As predicted, old males had higher mating success than young and middle-aged males in no-choice tests. Moreover, two-choice tests revealed that middle-aged males had a higher success rate than young males because of their higher courtship frequency rather than any female preference for them. It was found that old males produced a larger spermatophore than young and middle-aged males, suggesting greater reproductive effort. The protein content of spermatophores also tended to increase with male age. Despite the age-related variation in spermatophore size and protein content, age did not affect female fecundity or longevity. A decrease in the number of sperm in the older males might counteract the nutritional benefit of larger spermatophores. Alternatively, fitness components other than longevity and fecundity may be influenced by male age.