Although ADHD sometimes isn't discovered until adulthood, the disorder always begins in childhood and never spontaneously develops in adults. According to current diagnostic criteria, adults can't be diagnosed with adult ADHD unless it can be established they have had the disorder since they were at least seven years old. If you suspect you have adult ADHD but were never diagnosed as a child, it's possible for your medical experts to make a retrospective diagnosis through clinical interviews, physical examinations, and tests.