Introduction: Neurotrypsin (NT) is a neuronal trypsin-like serine protease whose mutations cause severe mental retardation in humans. NT is activated in vitro by Hebbian-like conjunction of pre- and postsynaptic activities, which promote the formation of dendritic filopodia via proteolytic cleavage of the proteoglycan agrin. Aims: 1) to examine the functional importance of NT for synaptic plasticity, learning, and extinction of memory in juvenile and matured mice, and 2) to evaluate the therapeutic potential of an NT-generated fragment of agrin, agrin-22. Methods: 1-month-old “Juvenile” NT−/− and NT+/+ mice were used for behavioral tests and LTP experiments; aged counterparts (18 months old) were used for behavioral tests and Golgi-cox staining based spine analysis; IHC based spine analysis for juvenile mice was performed in NT+/+/Thy1-EGFP-M+/* and NT−/−/Thy1-EGFP-M+/* mice; for intra-hippocampal AAV_agrin-22 injections, NT−/−/Thy1-EGFP-M+/* P7 mice were used. Results: Juvenile NT-/- mice display notable impairments in spaced LTP in CA3-CA1 synapses and deficits in contextual fear memory and sociability. The latter persists in aged NT-/- mice, which, unlike juvenile mice, show normal recall but impaired extinction of contextual fear memories. Morphological analyses revealed structural alterations in juvenile and aged NT-/- mice, including reduced spine density and changes in dendritic spine morphology in the hippocampal CA1 region. Agrin-22 administration in NT-/- juvenile mice increased the spine density, size and density of presynaptic boutons. Conclusions: These findings highlight NT's critical role in neurodevelopment and synaptic functions, and suggest new therapeutic options for targeting synaptic growth and improvement of cognitive functions.