In Huntington disease, the accurate determination of age-at-onset is critical to identify modifiers and therapies that aim to delay it.Retrospective data from the European Huntington's Disease Network's REGISTRY and PREDICT-HD, a longitudinal study in prodromal huntingtin gene expansion mutation carriers. Data (age, gender, CAG repeat length, parent affected, and Unified Huntington's Disease Rating Scale motor score, total functional capacity) from at least three visits in 423 REGISTRY and 124 PREDICT-HD participants were included. Data based extrapolations of individual age-at-onset using generalized linear mixed models based on individual slopes of motor score or total functional capacity, and predictions using the Langbehn, or Ranen formula, were compared with clinicians' estimates.Concordance was best for the observed age-at-onset in PREDICT-HD and the calculated onset using the PREDICT-HD UHDRS longitudinal motor scores. This was superior to the REGISTRY data. For total functional capacity, the investigator's estimate was 4 years before the data derived age-at-onset. The concordance of predictions of probability of age-at-onset is better with the observed age-at-onset in the PREDICT-HD data (difference in 25%tile -5 to 10 years) than the REGISTRY data (±20 years).Estimating or predicting age-at-onset in Huntington disease may be inaccurate. It can be useful to 1) add in the manifest population motor score regression derived age-at-onset as additional motor onset and 2) add total functional capacity regression derived age-at-onset for the onset of functional impact of Huntington disease when patients are in mid- to late-stage.