Plants need to balance investments in growth and defense throughout their life to increase their fitness. To optimize fitness, levels of defense against herbivores in perennial plants may vary according to plant age and season. However, secondary plant metabolites often have a detrimental effect on generalist herbivores, while many specialists have developed resistance to them. Therefore, varying levels of defensive secondary metabolites depending on plant age and season may have different effects on the performance of specialist and generalist herbivores colonizing the same host plants. In this study, we analyzed concentrations of defensive secondary metabolites (aristolochic acids) and the nutritional value (C/N ratios) of 1st-, 2nd- and 3rd-yearAristolochia contortain July (the middle of growing season) and September (the end of growing season). We further assessed their effects on the performances of the specialist herbivoreSericinus montela(Lepidoptera: Papilionidae) and the generalist herbivoreSpodoptera exigua(Lepidoptera: Noctuidae). Leaves of 1st-yearA. contortacontained significantly higher concentrations of aristolochic acids than those of older plants, with concentrations tending to decrease over the first-year season. Therefore, when first year leaves were fed in July, all larvae ofS. exiguadied andS. montelashowed the lowest growth rate compared to older leaves fed in July. However, the nutritional quality ofA. contortaleaves was lower in September than July irrespective of plant age, which was reflected in lower larval performance of both herbivores in September. These results suggest thatA. contortainvests in the chemical defenses of leaves especially at a young age, while the low nutritional value of leaves seems to limit the performance of leaf-chewing herbivores at the end of the season, regardless of plant age.