Adolescent and young adult (AYA) patients with acute leukemia (AL) have inferior outcomes in comparison to younger patients, and are more likely to develop acute and chronic GVHD than younger children following HLA matched sibling donor stem cell transplant (SCT). We compared the incidence of grade II–IV acute GVHD, chronic GVHD, and survival in AYA (age 13–21 years) to younger children (age 2–12 years) who received an unrelated donor SCT for acute leukemia on Children’s Oncology Group trials between 2004–2017. One hundred and eighty-eight children and young adults ages 2–21 years underwent URD SCT. Sixty-three percent were aged 2–12 and 37% were age 13–21. Older age was a risk factor for grade II–IV acute GVHD in multivariate analysis with a hazard ratio (HR) of 1.95 [95% confidence interval (CI) 1.23–3.10], but not for chronic GVHD, HR 1.25 [95% CI 0.57–2.71]. Younger patients relapsed more often (34.5 ± 4.4% vs. 22.8 ± 4.0%, p = 0.032), but their Event-Free Survival (42.6 ± 4.7% vs. 51.8 ± 6.1%, p = 0.18) and Overall Survival at 5 years (48.5 ± 4.9% vs. 51.5 ± 6.4%, p = 0.56) were not different than AYA patients. AYA patients who receive an URD SCT for acute leukemia are significantly more likely to develop grade II–IV acute GVHD, though survival is similar.