Personal anecdote, since there are some comments here by people claiming an early ADHD diagnosis might have changed their life: I was "diagnosed" with ADHD about five years ago, at around age 25, and it was a horrible experience. I'm not sure if I have ADHD, but at the very least was very depressed at the time, and they gave me some self-evaluation asking a bunch of questions like, "Do you have trouble concentrating? Does your mind wander?" Of course being very depressed I tended to be pretty hard on myself answering, they gave me the diagnosis, and at the first sign I gave of skepticism about it they basically ridiculed and humiliated me, telling me I was in denial and nothing in my life would ever change unless I started taking medication. They completely ignored any other problems I was having and other possible explanations once they pinned the ADHD label on me. In a conversation that lasted an hour, I didn't get a single word in after making the comment, "Well I'm not really sure if I have this..."

A few years later I did try taking some Adderall, unofficially acquired from a friend, and in three days it caused me to start having all kinds of mental health problems (OCD and Tourette's type symptoms, fyi) that I'm still not fully recovered from. Admittedly I think I took a bit too high of a dosage, but I'm scared to even try again if only three days of something could basically turn me halfway insane. Maybe if I'd started on only a half dose it wouldn't have happened, but I'm worried about the fact that such people were trying so hard to push a drug that ended up having those kinds of effects in only three days from only a slightly elevated dosage.

So I definitely think this whole thing is a huge pharma scam at this point. I'd be highly skeptical of any ADHD diagnoses, especially when they're all based on self-reporting (notoriously unreliable) and a bunch of therapists, teachers, etc. who are quick to jump to whatever explanation fits with their prejudices about a person. It scares me that they're pushing these drugs on people, especially at such young ages, not to mention how it prevents the real underlying problems from being solved.