The real debate here, just under the surface,is not whether a disorder "exists" or not. It certainly exists, because it is observed.
The real debate is just another iteration of the endlessly repeated nature vs. nurture debate. Is it a defect present at birth or is it acquired? And that, like all iterations of nature vs. nurture, soon becomes a logical circle. The fact that medication works on something does not tell us whether or not the condition is congenital or acquired by behavior.
If this "ability" of attention is normally formed in early childhood in response to external demands to pay attention to one thing and not another (read: parenting), and if a child were to simply not form this ability due to a lack of externally imposed discipline, then that lack of "ability" could easily be hardwired into the child as the developing brain bypasses that developmental stage and leaves a cognitive gap behind as it moves on to other developments.
This cognitive gap would look, by observing behavior at school age, just like a congenital defect. And medication would work.
The debate is fierce because the possible conclusions are so disparate: either there is a lot of bad parenting in the first 3 or 4 years of life, or we have discovered a condition which long was attributed to bad character.
Either a terrible injustice is being done on this generation of children by their parents, or a terrible injustice was done on earlier generations of children by their parents.
Just like athletic ability, it's probably some of both. The "nature" part is a physical tendency toward the symptoms (for example, a tendency toward inhibited dopamine uptake), and the "nurture" part might either reinforce an actual condition, or introduce a similar sort of behavior in those not physically pre-disposed toward it.
I know kids who fall fully on either side, where nature or nurture are probably completely dominant. I also know kids whose cases are somewhere in the middle. For example, with our daughter (who is very bright) we find that we can manage things by behavioral things (ours and hers), and by diet (we have to keep her away from red dyes). As parents we can guiltily point to things we could have done differently when she was little. But there's undoubtedly a strong "nature" component with her: she's pretty much exhibited the tendencies since birth.
I am quite certain that ADHD is too-often used as a convenient excuse. But that doesn't mean it's fake.
As I said at the outset, there's a powerful ideological component to the ADHD debate, as can be seen in this very thread. I have to say that those folks are probably as damaging to kids as are those who over-subscribe to ADHD diagnoses.
Not only do they loudly shout out piles of misinformation (i.e., "there's no such thing"), but they also make those on the "pro-ADHD" side appear to be calm and reasonable -- thereby strengthening the very thing they seek to get rid of. (Sean Hannity made a complete ass of himself on Friday with just such a claim, when he was discussing this article.)
I have no use for them.