Children who are the youngest in their class to be identified with ADHD are just as likely to keep the diagnosis as older pupils in their year group, scientists have found.
Experts from the University of Southampton and Paris Nanterre University, working with researchers worldwide, made the discovery after examining data from thousands of patients with attention deficit hyperactivity disorder.
In the past, scientists have questioned the validity of an ADHD diagnosis in younger pupils - arguing they receive it because they are less mature than those born towards the start of the school year.
But the study, published in Lancet Psychiatry, revealed children who are the youngest in the class and get diagnosed with the condition were still as likely to retain it later on as their older peers.
Senior lead author Professor Samuele Cortese , a child and adolescent psychiatrist at the University of Southampton, said: "We know the youngest children in their year group are more likely to be diagnosed with ADHD - but many believe this is because they lag behind their older classmates.
"However, no one has ever explored if these younger children who are diagnosed with ADHD retain the diagnosis later on - until now. Our study shows for the first time that these youngsters are no more likely to lose the diagnosis over time than older children."
Around 360 million people worldwide have been diagnosed with ADHD, according to the World Health Organisation, with around a third under the age of 18.
Symptoms include impulsiveness, disorganisation, poor time management skills, difficulty focusing and restlessness.
The study by Southampton and Paris Nanterre, undertaken with 161 scientists worldwide, was based on the largest data set ever created to explore the effect of month of birth on the persistence of ADHD.
In total, it examined data from more than 6,500 patients globally who have been followed up for a period between the ages of four and 33 years old.
Dr Corentin Gosling , an associate professor from the University Paris Nanterre in France and visiting researcher at Southampton, was the first author on the study.
He said: "Our work shows the diagnosis of ADHD in children with a young relative age is not especially unstable.
"However, it could not assess whether it is an appropriate diagnosis or it is because, once a child receives the ADHD label, parents and teachers consider the child as having ADHD and are influenced by the diagnosis. Future studies should solve this question."
Read the study in the Lancet Psychiatry.