I've only worked with adults who have ADHD. It's a very debilitating illness.
I actually diagnosed one of my friends as having that disease. I was helping her edit a paper for university, and her writing was incomprehensible. There was no flow to it at all. After asking about her writing, it became apparent that she had no idea how bad her writing was. She said she would finish a sentence without remembering how the sentence began. That, in a nutshell, is ADHD. This was the same friend who would remember things incorrectly and constantly misplace things. She would put post it notes on everything because it was the only thing to keep her focused on things that needed to be done. She would write class notes in multiple colors because it was the only way she could remember things. If she had a lot of work to do, she would put assignments on the floor and arrange the pieces of paper. She couldn't arrange lists in her head. She was absolutely terrible at estimating time and space. She would estimate something as taking 30 minutes when it would actually take more like 120 minutes. Her bedroom was a cluttered mess, and she didn't know where anything was. She would buy a journal, write a few pages, forget that she has a journal, and buy a new journal. If you asked her to describe what happened yesterday, her version of events was completely wrong. She wouldn't remember exact words or actions, but she would remember emotions. She could explain how she felt when someone said something, but she couldn't say exactly what was said. It caused a lot of communication problems.
The writing problem is what raised the red flag for me. I said she should get tested for ADHD. She did get tested, and the results were that she had little or no working memory. She was very intelligent, but she couldn't remember things. That's ADHD. There's lots of potential, but it's wasted because the person can't keep track of ideas in their head; they have no attention span. They can't sort lists in their head. They can't add numbers in their head. They can't do cross multiplication in their head. They can do it on paper through rote memorization, but they don't really understand what they are doing. In the mind of someone with ADHD, math class is just a series of meaningless steps. If you ask them the difference between sine and cosine, they can only think in terms of some rule they remembered. They can't think of the graphs in their head, think of how the situation works, and match the graph with the situation.
You might have dated someone who had ADHD. You would say something, they would get mad, and they wouldn't remember why they were mad. If you ask the exact words you said to make her mad, she would say "I don't remember it exactly, but I know you meant _____."
Boys get diagnosed more than girls because boys tend to be hyperactive whereas girls become inattentive. A boy with ADHD is more likely to have "discipline problems" and get put on drugs. A girl with ADHD is more likely to be spaced out, day dreaming all the time. Day dreaming doesn't bother anyone, so nobody notices the problem. My friend was the inattentive type. She could be sitting next to us, completely silent, and she would have no idea what we were talking about. Nobody would notice she wasn't paying attention until she was asked questions.