I'd never thought of this, but it sure sounds plausible. This study says one reason some kindergarteners are diagnosed as ADHD is that they are fidgetier than the other kids in class. One reason some kids are fidgetier than other kids in kindergarten is that they are a year younger than the oldest kids in class.
Kids born on December 1, who are the youngest kids in their kindergarten class California, are a year smaller and have a year less cognitive and social development than kids born on December 2 of the year before. This is why athletes and school leaders are disproportionately born in the early part of a school year, because that makes them a year bigger, smarter, and more coordinated than the kids born late in the cycle.
And the study linked to above says there may be a million kids misdiagnosed as ADHD. A million kids getting stimulants every day for years. I guess that's no worse than giving them bisphenyl A or organophosphates, which we do as a matter of government policy of acquiescing to corporate convenience. At least Ritalin doesn't screw up their sex hormones.
For the record, I was 4 years 6 months and 8 days on entering kindergarten; 4 years 6 months was the cutoff in California back in the Pleistocene. My mom had to go to work, and kindergarten was child care. She always regretted it. I was always glad. I wasn't the biggest, most coordinated, or most mature, but I was still the smartest. Although I guess it would be convenient to blame my character faults on it.