ADD People are smart

The vast majority of people with A.D.D are smart. They are also almost universally underachieved to the level of their intellect. Because they haven’t done well in school or frequently have had career failure and disappointments, which may negatively impact their self-esteem and render them feeling unsuccessful, still that does not make them any less intelligent. And many ADD people are very smart, so smart in fact that the early grades of school offer little or no challenge to their performance capacity. Either they can pay “enough attention to get it” or, as ADD people always end up doing, they develop successful (initially, at least) in-the-moment compensatory strategies to enable them to succeed. They cut corners, they wait till the night before and than “perform under pressure”, they find smart friends who take good notes in class, and sometimes they cheat. They adapt as they need to and do so as a functional adaptation (a “survival strategy” if you will).
For some, the strategy falls apart early, in elementary school (or even prior). Others sail through high school and undergraduate college and university (admittedly a smaller cohort). The largest single factor is often when the level of the intellect and the effectiveness of the compensations reach the breaking point related to the degree of difficulty and complexity of the tasks required. In other words, the functional capacity is no longer able to meet the demands and challenges of one’s life. It is not that one has “developed” ADD in your 20’s or 30’s; it is that your ability to adequately cope is no longer effective (“fraying at the seams” is the term I often use to describe it).
— Mayer Hoffer M.D., F.R.C.P.(C.), B.Sc.(Med.), Dip. Ch. Psych., Diplomate of the American Board of Psychiatry and Neurology Psychiatrist specializing in the diagnosis and treatment of ADHD across the life cycle