In Lepidoptera, it is known that larval and pupal testes enlarge in volume and then decrease in size over various phases of the pupal stage. After adult emergence, testis shrinkage continues. In the present study, the effects of temperature, age, and stage on testis development were investigated in the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae). Laboratory experiments indicated that in the adult stage, testis shrank markedly with age and with increasing temperatures, when males were reared at 15, 20, 25 or 30 °C during the larval and pupal stages and then transferred to other temperatures. When kept at 15, 20, 25, or 30 °C throughout their entire life (larva, pupa, and adult), the higher the temperature, the smaller the testis volume at any given adult age, indicating that larval and/or pupal rearing temperatures also affected testis development. Field experiments carried out in Tokyo showed that testis volume in wild adult males of diamondback moth was quite varied, suggesting that this population had both young and old males at each survey time. The lower the larval and pupal rearing temperature, the larger the adult, as measured by forewing length. Since larger adults tended to have larger testis at adult emergence, adult testis volume should be compensated based on the forewing length. Mating affected the testis size; testis volume decreased at higher mating frequency. Our results suggest that adult age of diamondback moths in wild populations can be estimated from the data on testis volume, temperature, and forewing length.