An important thing to understand about mental illness is that it's defined in relation to the society in which the patient lives. Some cultures may regard ADHD as a personality trait, and some cultures may recognize the cluster of symptoms as being a distinct "thing", but due to the way the society is set up it isn't a "disorder." For anything to qualify as a mental disorder it has to interfere significantly with your life.
I'm a good example. I sailed through high school and college without trying, earning decent grades just by paying attention to lectures and turning in reasonably good work on time. I was chronically disorganized, but thought of it as a personality trait; my thoughts are structured in a weird way and I make connections between things that others don't see. I thought I was just weird.
Well, around 30 I entered grad school and went to a counselor because I was extremely stressed and overwhelmed, and wanted to figure out how to cope with that. Also I had some historical shit to work out. But, when I was describing my situation he suggested ADD as a possibility and referred me out for diagnosis. Sure enough, I've got ADD-PI, the non-hyperactive type of ADD. Although I can see it everywhere in my life up to now, it was never clearly the cause of my problems earlier on. I was able to brute force my way through school on raw intelligence and an ability to sponge up info from lectures.
That wasn't enough for my current program, which has us taking 7 classes at once in topics ranging from "Pathology of musculoskeletal disorders" or "Clinical management of cardiopulmonary disorders" to statistical analysis & how to read an academic paper. There was one day when we started the morning learning massage techniques and ended the day with an overview of medications for diabetes. There's no way in hell a person with undiagnosed ADD can do well without other