The youngest classmates before age 10 were more likely to have ADHD.
In a Finish population-level study, the youngest classmates in early elementary school were more likely to receive a diagnoses of ADHD until 10 years of age. Confounders such as learning disorders did not affect the finding. Ellen G White, an early health reformer wrote, "It is customary to send very young children to school.... This course is not wise. A nervous child should not be overtaxed in any direction." (Child Guidance, page 302)
PositiveTip: It is healthier for a child to start school a little older rather than "outgrow" an ADHD diagnoses.