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.
As to the ADHD,
I also saw an article about a study saying that starting kindergarten early is correlated with increased ADHD (but I don't have access to the original study, so I don't know whether it's a causal relationship or just a correlation -- e.g., gifted kids are more likely to both have ADHD and start kindergarten early -- does anyone have more info on this study?)
I think your critical thinking here is right on: is this correlation or causation?
Two additional things to consider:
*If you're referring to the study I've read, the study is not in gifted kids, but comparing ADHD diagnosis rates to the relative age of a child across the entire population of kids. Those kids that are young for grade are diagnosed at a higher rate than those who are young for grade. There wasn't enough data to separate out those who were in fact younger than the youngest expected age in each grade.
*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.
But these are population studies, and not an analysis or in any way predictive for your child. For that you should use the IAS. If you are particularly concerned about ADHD, what do you see with regards to attention and executive skills in this child? How does this child function with older kids?
I would also look into the age distribution of incoming kindergarteners in this school. Here, the de facto cutoff is about 3-4 months before the district actual cutoff, so my grade accelerated kid (November bday) is more than 5 months younger than the next youngest boy in his class, and 20 months younger than the oldest kid in his class.
