Their executive functions will finish developing somewhere between the age of 25-30, so they will find it easier to cope with typical ADHD challenges, and hormonal levels will be more stable. Importantly, as they grow older, they might get better at coping with their symptoms. This in turn could lead to less distress and fewer challenges with daily functioning.
Children don’t truly “outgrow it” as this isn’t exactly possible. For example, in adults, hyperactivity might look more like mental restlessness, or a person’s fidgeting is more subtle. Another example: Children often experience their parents’ divorce in real time as somehow their fault, and thus may harbor guilt and self-doubt related to the event even many years later. Observing the events of a divorce from an adult perspective allows the client to realize that their parents’ divorce was not their fault, and that the childish expectation that their behavior could somehow have mended their parents’ rift was both development.