That's very insightful, but it's my guess is that it's not as salient as one would think. Scientists (at least in the lab) will typically use a number of metrics to measure ADHD, most notably cerebral myelination. ADHD tends to involve improper development in this regard (i.e., the brain does NOT myelinate as fast as one would expect). Since the authors posit that younger children may simply be less mature, my suspicion is that they are talking about this. And the obvious thing to do would be to correct for what you talk about (in fact, it's hard to see how they could get into a journal and not do so).