The gap between the oldest and youngest children in the class had a small, but significant, association with the increased use of ADHD medications. The researchers say their findings compare with those of other international studies.

It's possible the youngest children in a school year may find it harder to keep up in lessons than children almost a year older than them, and may be more likely to have problems with concentration.

But it would be a big assumption to say ADHD is being overdiagnosed and overtreated on the grounds of this study alone.