Aging is characterized by progressive memory decline that can lead to dementia when associated with neurodegeneration. Here, we show in mice that aging-related memory decline involves defective biogenesis of microRNAs (miRNAs), in particular miR-183/96/182 cluster, resulting from increased protein phosphatase 1 (PP1) and altered receptor SMAD (R-SMAD) signaling. Correction of the defect by miR-183/96/182 overexpression in hippocampus or by environmental enrichment that normalizes PP1 activity restores memory in aged animals. Regulation of miR-183/96/182 biogenesis is shown to involve the neurodegeneration-related RNA-binding proteins TDP-43 and FUS. Similar alterations in miR-183/96/182, PP1, and R-SMADs are observed in the brains of patients with amyotrophic lateral sclerosis (ALS) or frontotemporal lobar degeneration (FTLD), two neurodegenerative diseases with pathological aggregation of TDP-43. Overall, these results identify new mechanistic links between miR-183/96/182, PP1, TDP-43, and FUS in age-related memory deficits and their reversal.