The gut-brain axis is a bidirectional communication pathway that modulates cognitive function. A dysfunctional gut-brain axis has been associated with cognitive impairments during aging. Therefore, we propose evaluating whether modulation of the gut microbiota through fecal microbiota transplantation (FMT) from young-trained donors (YT) to middle-aged or aged mice could enhance brain function and cognition in old age. Twelve-month-old male mice received an initial FMT from YT (YT-Tr) or age-matched donors (Auto-Tr) following antibiotic treatment. Three months later, the mice received a second FMT as reinforcement. Additionally, 18-month-old mice received Auto-Tr, YT-Tr, or FMT from young sedentary donors (YS-Tr). Cognitive function was assessed using novel object recognition and object location memory tests. Long-term potentiation (LTP) in hippocampal brain slices was studied, while neuroinflammation and synaptic plasticity were analyzed in hippocampal samples via qPCR and immunoblot. Gut permeability was evaluated in ileum and colon sections, serum samples were analyzed for cytokine levels, and fecal samples were used to measure short-chain fatty acid (SCFA) levels and perform 16S rRNA gene sequencing. We observed that YT-Tr, whether performed in middle age or old age, improved cognitive function in aged mice. Recognition and spatial memory were significantly enhanced in YT-Tr mice compared to Auto-Tr and YS-Tr groups. Intact LTP was observed in YT-Tr mice at 18 months of age, whereas LTP was impaired in the Auto-Tr group. Neuroinflammation was reduced, and synaptic plasticity modulators such as PSD-95 and FNDC5/Irisin were upregulated in the hippocampus of YT-Tr mice compared to both YS-Tr and Auto-Tr groups. A significant reduction in ileal and colon permeability was detected in YT-Tr animals, along with elevated cecal levels of butyrate and valerate compared to Auto-Tr. Moreover, YT-Tr decreased pro-inflammatory factors and increased anti-inflammatory factors in the serum of aged mice. Beta diversity analysis revealed significant differences in microbial community composition between YT-Tr and Auto-Tr animals, with higher abundances of Akkermansia, Prevotellaceae_UCG-001, and Odoribacter in YT-Tr mice. In conclusion, our study demonstrates that FMT from young-trained donors improves cognitive function and synaptic plasticity by modulating gut permeability, inflammation, SCFA levels, and gut microbiota composition in aged mice.