Group 2 innate lymphoid cells (ILC2s) in mouse lungs are activated by the epithelium-derived alarmin IL-33. Activated ILC2s proliferate and produce IL-5 and IL-13 that drive allergic responses. In neonatal lungs, the occurrence of spontaneous activation of lung ILC2s is dependent on endogenous IL-33. Here, we report that neonatal lung ILC2 activation by endogenous IL-33 has significant effects on ILC2 functions in adulthood. Most neonatal lung ILC2s incorporated 5-bromo-2'-deoxyuridine (BrdU) and persisted into adulthood. BrdU+ ILC2s in adult lungs responded more intensely to IL-33 treatment compared with BrdU- ILC2s. In IL-33-deficient (KO) mice, lung ILC2s develop normally, but they are not activated in the neonatal period. Lung ILC2s in KO mice responded less intensely to IL-33 in adulthood compared with WT ILC2s. While there was no difference in the number of lung ILC2s, there were fewer IL-13+ ILC2s in KO mice compared with those in WT mice. The impaired responsiveness of ILC2s in KO mice was reversed by i.n. administrations of IL-33 in the neonatal period. These results suggest that activation of lung ILC2s by endogenous IL-33 in the neonatal period may "train" ILC2s seeding the lung after birth to become long-lasting resident cells that respond more efficiently to challenges later in life.