In model systems, studies have indicated that aged brains can be rejuvenated by certain systemic interventions, such as the transfer of plasma from young animals to old animals. The cerebrospinal fluid (CSF) surrounds the brain, but it was not known whether young CSF can exert similar effects. Now, Iram et al. show in mice that infusions of young CSF into the brains of aged animals promote oligodendrogenesis and improve memory function, and that fibroblast growth factor 17 (FGF17) is a key molecule that mediates these effects.