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Adult newborn neurons are involved in memory encoding and extinction, but the neural mechanism is unclear. We found the adult newborn neurons at 4 weeks are recruited by learning and subjected to epigenetic regulations, consequently reducing their ability to be re‐recruited later. After removal of the epigenetic blockage, Suv39h1 KO mice showed an increased recruiting number of aged newborn neurons and enhanced flexibility in learning tasks. Besides NRXN1, we found SHANK1, the synaptic scaffold protein, is one of the major targets of Suv39h1, regulating memory stability. Expression of Shank1 is transiently engaged to enhance synaptogenesis during learning and is strongly suppressed by Suv39h1 from 5 h after learning. Exogenously overexpression of Shank1 in dentate gyrus increased the density of mushroom spines and decreased the persistency of old memories. Our study indicated the activity‐regulated epigenetic modification in newly matured newborn neurons in hippocampus insulates temporally distinct experiences and stabilizes old memories. [ABSTRACT FROM AUTHOR]