This paper is a commentary by one researcher. He reviewed two 2016 follow-up studies, one from Brazil and one from the UK, which reported that many young adults with ADHD had shown no ADHD as children. He argues these results do not yet prove that a separate adult-onset form of ADHD exists.
He points to how the symptoms were counted. Childhood ADHD was rated by parents, but young-adult ADHD was rated by the people themselves, and self-reports tend to run much lower. In his own Milwaukee follow-up, only a small share of young adults reported enough symptoms for ADHD (4%), while parents reported far more (46%), and the figure rose higher with an age-adjusted definition (66%). A study of girls showed the same gap (22% by self-report versus 44% by parent report).
He also argues that many so-called late-onset cases had mild symptoms in childhood that later crossed the cutoff, and that people recall their age of onset poorly, sometimes off by 5 to 7 years. He accepts that ADHD can follow brain injury and says more evidence is needed.
The author suggests that clinicians should not place too much faith in the exact age-of-onset cutoffs in recent DSM editions, and that significant symptoms causing real-life impairment matter more.
Source: Barkley R. Is There an Adult Onset Type of ADHD? Issues in Establishing Persistence and Remission of ADHD from Childhood to Adulthood. The ADHD Report (2016). doi:10.1521/adhd.2016.24.6.6