Leptoglossus occidentalis Heidemann (Heteroptera, Coreidae) is native to Western North America and is a serious pest for seed production of conifers. The pest was accidentally introduced into Europe in the 1990s. Since then, seed loss has been recorded in Pinuspinea (L.) forests, with a negative impact on the commercial production of pine nuts. Classical biological control of this pest in P. pinea stands is an attractive proposition. Previous work showed that the egg-parasitoid Gryon pennsylvanicum (Ashmead) (Hymenoptera, Platygastridae) had promising life history traits in laboratory studies using L. occidentalis eggs as host. In the present work, the effect of host egg age on parasitization rate was evaluated in the laboratory, using choice and no-choice tests. Host eggs ranged in age from <24 h to within a day of hatching. Results showed that parasitization rate, juvenile survival rate, sex ratio, and longevity of female G. pennsylvanicum were not significantly affected by the age of the host eggs. However, egg-parasitoid development time was longer in older host eggs, and females were smaller than those that developed in younger host eggs. Parasitization behaviors (drumming, oviposition, and marking) were not affected by the age of the host. G. pennsylvanicum females tended to parasitize all available host eggs within a cluster before moving to a new cluster, without displaying a preferences for host egg age. The ability to exploit host eggs of any age class improves the prospect of successful classical biological control using this egg-parasitoid.