ADHD diagnosis revolves around school. When you read the definition in the psychiatry books, it boils down to this -- The child can't sit still during a boring class.
I homeschool/unschool. I KNOW that my 11-yr-old son is healthy and normal, but his right-brained personality will not sit still for anything boring or irrelevant to him. When he really wants to learn something, he can sit still for hours! So it's not a matter of not being able to focus -- just a refusal to focus on boring stuff. That's his personality, as it is for a lot of boys especially. It's normal. It's not a disorder. It is the school system that needs to change, not the child.
On the other hand, my son does have one friend that they have labelled ADHD that I feel does have a disorder of some kind. He can drive me and others a bit nuts even in the home setting where children are playing as they wish. Still, I think that the vast majority of ADHD diagnosis is simply trying to fit an active, strong-willed child into a little box during school hours.