Background: Early identification of developmental disabilities is advocated, so that affected children and families can be adequately supported. There is a lack of data on factors associated with earlier diagnosis in four of the most common neurogenetic syndromes. Aims: The aim was to identify the mean ages of diagnosis in four of the most common neurogenetic syndromes and identify factors associated with early diagnosis. Method: The parents of school‐aged children (4–19 years) with one of the four genetic syndromes (Fragile X syndrome, Prader–Willi syndrome, Williams syndrome, 22q11.2 deletion syndrome (22qdel)) in the UK and Ireland were surveyed and asked about age of diagnosis and the presence of a range of medical conditions. Findings: Mean age of diagnosis was lowest in Prader–Willi syndrome (0.47 years), and this was significantly lower than Fragile X syndrome (3.52 years), Williams syndrome (1.76 years), and 22qdel (3.21years) (all p < 0.001). Factors associated with diagnosis before 3 years in 22qdel included (1) the presence of a cardiac difficulty (OR 4.9 (95% CI 1.1–21.0)) (2) hearing difficulty (OR 2.5 (95% CI 2.01–70.8)), and (3) chronological age <12 years (OR 9.1 (95% CI 1.8–46.0)). The only factor significantly associated with diagnosis before 18 months in Williams syndrome was the presence of a cardiac difficulty (OR 2.9 (95% CI 1.1–8.3)). None of the assessed predictors was associated with a diagnosis of Fragile X syndrome before 3 years of age. Discussion: Mean age of diagnosis differed significantly in the four neurogenetic syndromes. While the presence of medical factors is likely to lead to earlier diagnosis in 22qdel and Williams syndrome, factors associated with earlier diagnosis in Fragile X syndrome were not identified. Delays in diagnosis could have significant implications with respect to understanding of children's needs and nature and level of support accessed by affected children and their families.