"Anywhere in which age cutoffs arbitrarily divide kids into different groups a situation will be generated in which some are older than others—sometimes significantly. The most obvious example of this is in school. When a state dictates that a kindergarten entrant be “five years old before September 1,” that kindergarten class is going to be composed of kids whose ages span as much as a year. A child born on August 31 will be 364 days younger than one born on September 1, even though both will belong in the same class.
At that early age, one additional year of life is no small thing. The oldest kids in the kindergarten class could have as much as 20 percent more experience here on planet Earth, not to mention the accompanying physical growth, than the youngest kids in the same class. Teachers and school systems, however, apply the same set of expectations for all kids in that class, regardless of the month of their birth. They’re taught the same lessons, evaluated by the same metrics, and expected to behave the same way...
As with influenza, there should be no obvious biological differences in the risk of ADHD based simply on a child’s birthday. So we could group kids by their birth month and assume that they were counterfactual to each other; that is, what happened to the group of kids born in August is what would have happened to the kids born in September had they been born in August, and vice versa. If we saw differences in ADHD rates between kids born in one month versus the other, we could deduce that they were the result of external factors, and not some biologically inherent feature...
Here’s what we found: in states with a September 1 cutoff, kids born in August had a 34 percent higher rate of ADHD diagnosis and treatment than those in the same class born in September of the previous year...
There are other ways the relative age effect can manifest itself in our health. A study from the U.K. showed that the youngest kids in a school class year were not only more likely to be diagnosed with ADHD and intellectual disability but also more likely to be diagnosed with depression. A review of data in Alberta, Canada, from 1979 to 1992 showed that among individuals under twenty who committed suicide, a disproportionate share (55.3 percent) were in the younger half of their school class. A study in Norway, meanwhile, found that being young for your grade was associated with increased probability of teenage pregnancy.
The relative age effect has been shown to affect classroom outcomes beyond a child’s health. In a study of children in England with a school entry cutoff of September 1, researchers found that standardized test scores were consistently lower for boys and girls born in August compared with their older, September-born peers. The size of the effect, which they called the “August birth penalty,” was most pronounced in the youngest kids they studied, the five-year-olds, though it was present as old as age eighteen. After accounting for other potential factors (such as when other kids in a given class were born, when in the school year kids entered their class, or whether kids switched schools), the researchers came to the conclusion that “the major reason why August-born children perform significantly worse than September-born children in the Key Stage tests is simply that they are almost a year younger when they sit [for] them.”...
Compared with September-born kids, the August-born kids were also more likely to be diagnosed with behavioral, cognitive, or physical disabilities, and more likely to be in remedial reading or math courses. August-born kids were less likely to be enrolled in programs for gifted students, less likely to be enrolled in advanced reading or math courses, less likely to be enrolled in Advanced Placement courses, and less likely to graduate from high school on the standard schedule. By and large, the effects remained even after controlling for gender, race, and maternal level of education."
--- Random Acts of Medicine: The Hidden Forces That Sway Doctors, Impact Patients, and Shape Our Health / Anupam B. Jena & Christopher Worsham
