ADHD is the result of a deficiency in the dopaminergic system inside the brain. It is real, but grossly overdiagnosed.
ADHD has nothing to do with a "deficit" of attention, in reality. It is actually an overabundance of attention, coupled with a lack of effective filtering to eliminate secondary and background "noise" that distracts from the task at hand.
There is a clinical method for diagnosing the disease that shows fully 50% of behaviorally diagnosed cases are false diagnoses. For a child with the disorder, ritalin and its chemical family help make up for the deficiency in the dopamine handling system (when taken in a properly titrated dosage). For kids who are wrongly diagnosed, it turns them into zombies.
The above is fact. What follows is only my opinion.
The trend in diagnosing ADHD follows closely the introduction, promulgation, and elaboration of the "enrichment" theories of education. When I was in elementary school, the classroom was a rather dull place where one had to concentrate on paying attention. The "background noise" was deliberately reduced to enhance each child's ability to "pay attention". Those whose attention wandered often paid for it with embarrassment (or sore knuckles, if you had the ruler-wielding samurai type of teacher).
Today, the exact opposite educational theory is in play. Elementary school education is rife with "enrichment", adding visual and aural noise into the environment until formulation of an effective attention filter (for those with ADHD) becomes too difficult.
That's my $.02, anyway.
IOW, distraction and overstimulation. As an adult I keep my house relatively empty because it isn't just audio noise it is also visual "noise". I work in an office where there are 4 or 5 people and people in and out, I have to go in after hours to get any real work done.