Known sometimes as the "birthdate effect" or "relative age effect", this generally diminishes as children get older but does not vanish.

BBC: Month of birth affects chance of attending Oxbridge

"With this data now available from more than one study, it really becomes the responsibility of parents, physicians, and teachers to pay attention to the relative-age effect and not intervene with treatment and labeling of children who it's no fault of their own what time of year they were born, " Garland says.

CNN: Youngest kids in class more likely to be diagnosed with ADHD