This study examined differences in referral and diagnosis based on sex recorded at birth (hereafter, 'sex'), using case notes from 408 individuals diagnosed by 30 multidisciplinary teams across Scotland. Analyses focused on male-to-female ratios and median ages at referral and diagnosis across attention-deficit/hyperactivity disorder, autism and intellectual disability. The lifespan male-to-female ratio across all diagnoses was 1.31, varying by category and age. In autism, the male-to-female ratio was 2.21 for children under 10 years, but there were more females in adolescence (male-to-female ratio = 0.79) and adulthood (male-to-female ratio = 0.94). Across the lifespan, combining all diagnoses, females were referred later (median ages: 14.4 vs 19.7 years; p < 0.001) and diagnosed later (median ages: 15.2 vs 20.2 years; p < 0.001), indicating a 5-year delay. Among autistic children and adolescents, females were referred later (median age: 7.5 vs 10.5 years; p = 0.002) and diagnosed later (median age: 9.3 vs 11.9 years; p = 0.003). However, no significant differences were found in age of referral or diagnosis for autistic adults. Overall, the results indicate increasing balance in sex ratios with age, greater asymmetry in younger age groups, and consistent delays in referral and diagnosis ages for females.Lay abstractThis article looks at how males and females are diagnosed with neurodevelopmental differences including autism, attention-deficit/hyperactivity disorder (ADHD) and intellectual disabilities. It studies the case notes of 408 people (adults and children) assessed by 30 different teams during their diagnosis process. The results show overall that more males were diagnosed with neurodevelopmental differences and that males are diagnosed at younger ages compared to females. The increased diagnosis of males compared to females is most common in younger ages, but and as people get older, the number of males and females diagnosed becomes more equal, showing that more women are coming forward for and being diagnosed, for example with autism and ADHD. This research shows that although there are increasing amounts of females diagnosed, that girls and women are still missed when they are young, with more boys gaining a diagnosis as children. It seems there is a pattern of females being diagnosed at later ages (meaning less access to support and self-understanding). The study calls for better diagnostic practices and more research to address these gender differences.