Updated project metadata. We report age-differential synaptic plasticity deficits associated with cognitive inflexibility and CaMKIIa hyperactivity in Adnp-mutant mice. These mice show impaired and inflexible contextual learning and memory along with social and anxiety-related deficits in adults, long after the juvenile-stage decrease of ADNP protein levels to ~10% of the newborn level. Adnp-mutant mice show abnormally enhanced long-term potentiation in adults but not in juveniles. This is accompanied by CaMKIIa hyperactivity involving increased baseline autophosphorylation and altered phosphorylation of CaMKIIa substrates, as revealed by unbiased proteomic analyses.