Because ADHD is neurodevelopmental (i.e. dependent on brain development) it's not possible to make an accurate assessment until the age of five or six years. This is when the type of behaviours, often seen in those with ADHD, would be considered grossly immature for their age and a significant departure from typical neurodevelopment.
Data from the Ministry of Health suggests 2.4% of those 18 years and younger have had an ADHD diagnosis.