Introduction Medulloblastoma is the most common malignant brain tumor in children, but accounts for only 1% of brain cancers in adults. For standard-risk pediatric medulloblastoma, current therapy includes craniospinal irradiation (CSI) at reduced doses (23.4 Gy) associated with chemotherapy. Whereas most same-stage adult patients are still given CSI at 36 Gy, with or without chemotherapy, we report here on our use of reduced-dose CSI associated with chemotherapy for older patients. Methods We gathered non-metastatic patients over 18 years old (median age 28 years, range 18–48) with minimal or no residual disease after surgery, no negative histological subtypes, treated between 1996–2018 at the Centre Léon Bérard (Lyon) and the INT (Milano). A series of 54 children with similar tumors treated in Milano was used for comparison. Results Forty-four adults were considered (median follow-up 101 months): 36 had 23.4 Gy of CSI, and 8 had 30.6 Gy, plus a boost to the posterior fossa/tumor bed; 43 had chemotherapy as all 54 children, who had a median 83-month follow-up. The PFS and OS were 82.2 ± 6.1% and 89 ± 5.2% at 5 years, and 78.5 ± 6.9% and 75.2 ± 7.8% at ten, not significantly different from those of the children. CSI doses higher than 23.4 Gy did not influence PFS. Female adult patients tended to have a better outcome than males. Conclusion The results obtained in our combined series are comparable with, or even better than those obtained after high CSI doses, underscoring the need to reconsider this treatment in adults.