The article says Denmark:<p>&quot;For the study, researchers used data from a nationwide mental-health screening tool of children in Denmark – a survey that is widely used internationally in clinical and academic settings – and matched it against the Danish census. Linking the two allowed researchers to make analyses using exact dates of birth, allowing for robust comparisons among similarly aged children.<p>Because children in Denmark enroll in kindergarten in the calendar year they turn six, kids born exactly on Dec. 31 would have started kindergarten earlier that year, while those born a day later on Jan. 1 would be 6-years-and-8-months old when they start formal schooling.<p>Researchers were thus able to examine differences between children who were born a certain number of days before the Dec. 31 enrollment cut-off and those born after the cut-off. They used full-sample sizes from the parent-reported mental health survey; 54,241 parents responded when their children were age 7 and 35,902 responded when the children were about 11 years old.&quot;