Look at the Iowa Acceleration Scale for guidance on early entry to kindergarten. It's the only evidence-based tool out there to assist with this decision.
THIS, but you'll have to get on having appropriate tests run if you don't have them already. You can figure out the IAS recommendation using the IAS Manual and the appropriate tests.
*I think there's an additional implication of the study that you're missing: that the ADHD diagnoses are in fact misdiagnoses of immaturity rather than an actual disorder.
And this, absolutely. It makes the most sense in terms of the study results and other known problems with ADHD identification and schools.
FWIW my son skipped Kindergarten and entered 1st grade early (late October birthday). He legitimately has ADHD and is ALSO the youngest in his grade, while simultaneously working at a high level for academics. He has 2 classroom teachers job-sharing his class of 27 kids this year (so only 1 at a time, but on different days) and one of them has been just great with him and has glowing things to say; and one of them has complained all year about his "immaturity". He's not immature, he's just impulsive, but his teacher truly doesn't get the difference.