The youngest kids in the class who are diagnosed with ADHD actually do have ADHD, as they are no more likely to lose their diagnosis over time.
Studies have found that the youngest kids in a class are more likely to be diagnosed with ADHD, possibly because they lag behind their older classmates in maturity and self-control.
ADHD doesn't come and go; someone who has it, has it for life, so if a child loses their diagnosis, they were likely misdiagnosed.