ADHD researcher here, this was already known. ADHD is a neurodevelopmental disorder, the key is that developmental disorders occur over the time-course of typical development. To have a diagnosis of ADHD, there must have been symptoms before the age of 7, and furthermore, it's much more likely to occur in younger children because their executive functions are less well developed.
On the neurological level, there are different dopaminergic systems developing throughout childhood, such as the motor cortex, the prefrontal cortex, and the dopamine-based reward system. If the motor cortex develops too quickly before the PFC, which controls/regulates behavior, well you'll have hyperactivity that's unregulated. With less developed EF's comes getting out of the chair, losing things, daydreaming, hyperactivity, and all of the diagnosable symptoms of ADHD. Naturally, those who are 6-9 months younger (which might be a 10% age difference in kindergarten) are going to be in very different developmental stages. Requiring younger children to sit still and pay attention may be a much more challenging task since their brains are actually less developed. Thus, they more easily get pushed over the diagnosis line compared to their older classmates.
There is another wrinkle here beyond age, which is gender: boys are around 2.5-3X as likely to be diagnosed because they are more hyperactive naturally, i.e. their neurological systems develop at different rates than girls. This sort of behavior in the classroom is much more disruptive than what girls exhibit. Girls tend to be diagnosed with primarily inattentive subtype, since their motor cortex is not usually as over-active, but they still may suffer underdevelopment of the EF that helps them to focus. A girl daydreaming is much less disruptive than a boy standing on his chair, rolling on the floor, or wandering the classroom (all of which happened multiple times in my own classroom).
On the neurological level, there are different dopaminergic systems developing throughout childhood, such as the motor cortex, the prefrontal cortex, and the dopamine-based reward system. If the motor cortex develops too quickly before the PFC, which controls/regulates behavior, well you'll have hyperactivity that's unregulated. With less developed EF's comes getting out of the chair, losing things, daydreaming, hyperactivity, and all of the diagnosable symptoms of ADHD. Naturally, those who are 6-9 months younger (which might be a 10% age difference in kindergarten) are going to be in very different developmental stages. Requiring younger children to sit still and pay attention may be a much more challenging task since their brains are actually less developed. Thus, they more easily get pushed over the diagnosis line compared to their older classmates.
There is another wrinkle here beyond age, which is gender: boys are around 2.5-3X as likely to be diagnosed because they are more hyperactive naturally, i.e. their neurological systems develop at different rates than girls. This sort of behavior in the classroom is much more disruptive than what girls exhibit. Girls tend to be diagnosed with primarily inattentive subtype, since their motor cortex is not usually as over-active, but they still may suffer underdevelopment of the EF that helps them to focus. A girl daydreaming is much less disruptive than a boy standing on his chair, rolling on the floor, or wandering the classroom (all of which happened multiple times in my own classroom).