ABSTRACTHuntington disease is an autosomal dominant inherited brain disorder that typically becomes manifest in adulthood. Juvenile‐onset Huntington disease refers to approximately 5% of patients with symptom onset before the age of 21 years. The causal factor is a pathologically expanded CAG repeat in the Huntingtin gene. Age at onset is inversely correlated with CAG repeat length. Juvenile‐onset patients have distinct symptoms and signs with more severe pathology of involved brain structures in comparison with disease onset in adulthood. The aim of this review is to compare clinical and pathological features in juvenile‐ and adult‐onset Huntington disease and to explore which processes potentially contribute to the observed differences. A specific focus is placed on molecular mechanisms of mutant huntingtin in early neurodevelopment and the interaction of a neurodegenerative disease and postnatal brain maturation. The importance of a better understanding of pathophysiological differences between juvenile‐ and adult‐onset Huntington disease lies in development and implementation of new therapeutic strategies. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society