Key points The membrane‐associated guanylate kinase (MAGUK) family of synaptic scaffolding proteins anchor glutamate receptors at CNS synapses. MAGUK removal via RNAi‐mediated knockdown in the CA1 hippocampal region in immature animals causes rapid and lasting reductions in glutamatergic transmission. In mature animals, the same manipulation has little acute effect. The hippocampal dentate gyrus, a region with ongoing adult neurogenesis, is sensitive to MAGUK loss in mature animals, behaving like an immature CA1. Over long time courses, removal of MAGUKs in CA1 causes reductions in glutamatergic transmission, indicating that synapses in mature animals require MAGUKs for anchoring glutamate receptors, but are much more stable. These results demonstrate regional and developmental control of synapse stability and suggest the existence of a sensitive period of heightened hippocampal plasticity in CA1 of pre‐adolescent rodents, and in dentate gyrus throughout maturity. Abstract Fast excitatory transmission in the brain requires localization of glutamate receptors to synapses. The membrane‐associated guanylate kinase (MAGUK) family of synaptic scaffolding proteins is critical for localization of glutamate receptors to synapses. Although the MAGUKs are well‐studied in reduced preparations and young animals, few data exist on their role in adult animals. Here, we present a detailed developmental study of the role of the MAGUKs during rat development. We first confirmed by knockdown experiments that MAGUKs are essential for glutamatergic transmission in young animals and cultured slices, and an increase in postsynaptic density protein 95 (PSD‐95) by overexpression caused correlated increases in glutamatergic transmission. We found that CA1 synapses in adults, in contrast, were largely unaffected by overexpression of MAGUKs, and although adult CA1 synapses required MAGUKs to the same degree as synapses in young animals, this was only apparent over long time scales of knockdown. We additionally showed that overexpression of MAGUKs is likely to function to accelerate the developmental strengthening of excitatory transmission. Finally, we showed that adult dentate gyrus appears similar to immature CA1, demonstrating regional and developmental control of MAGUK dynamics. Together, these results demonstrate a period of juvenile instability at CA1 synapses, followed by a period of adult stability in which synapses are acutely unaffected by changes in MAGUK abundance.