IIRC, the study made no mention of giftedness or even academic ability of any of the children at all. They simply made a big deal of the utterly banal fact that less mature kids will have less mature executive functioning skills than more mature kids as compared to their average classmates. If your kid has executive functioning weaknesses and they are the youngest in the classroom, chances are higher they might be diagnosed with ADHD because they can't function, and maybe have academic weaknesses as well which may mimic ADHD, too (ie they are squirming and acting up not because they have ADHD but because they are in way over their head) and parents or teachers are pushing for Evals and diagnoses because the kid is struggling. If they have EF weaknesses and are among the oldest kids in the classroom, chances are they have better compensation skills and/or are functioning better because academics are easier for them, ie no ones pushing for a diagnosis because the kid is muddng along just fine. But they're the same kid, just the reference group is a year older or younger and standards are more or less strict.
There are very few studies that make reference about gifted kids just because there are very few gifted kids and it's super hard to get randomized samples of accelerated/non accelerated gifted kids. Those that do exist point to beneficial effects. After all, you WANT the gifted kids to be with the older peer group because you WANT the academic challenge, and the question is whether a kids lack of socio emotional maturity might be a deal breaker.
