Rett syndrome (RTT) is a postnatal neurological disorder caused by mutations in MECP2, encoding the epigenetic regulator methyl-CpG-binding protein 2 (MeCP2). The onset of RTT symptoms during early life together with findings suggesting neurodevelopmental abnormalities in RTT and mouse models of RTT raised the question of whether maintaining MeCP2 function exclusively during early life might protect against disease. We show by using an inducible model of RTT that deletion of Mecp2 in adult mice recapitulates the germline knock-out phenotype, underscoring the ongoing role of MeCP2 in adult neurological function. Moreover, unlike the effects of other epigenetic instructions programmed during early life, the effects of early MeCP2 function are lost soon after its deletion. These findings suggest that therapies for RTT must be maintained throughout life.