Adult attention deficit hyperactivity disorder is ADHD that persists into adulthood. It is a neurodevelopmental disorder, meaning impairing symptoms must have been present in childhood, except for when ADHD occurs after traumatic brain injury. According to the DSM-5 diagnostic criteria, multiple symptoms should have been present before the age of 12. This represents a change from the DSM-IV, which required symptom onset before the age of 7. This was implemented to add flexibility in the diagnosis of adults. ADHD was previously thought to be a childhood disorder that improved with age, but later research challenged this theory. Approximately two-thirds of children with ADHD continue to experience impairing symptoms into adulthood, with symptoms ranging from minor inconveniences to impairments in daily functioning, and up to one-third continue to meet the full diagnostic criteria.