It was found that children born in December, so the youngest in their school year, were 40 per cent more likely to be diagnosed and almost 50 per cent more likely to be treated for ADHD than children born in January, the start of the school year.
In Britain this would correspond to children born in August compared with those born in mid to late September.
It is well known that the youngest children in the school year tend to struggle with academic performance, behaviour and sport, in what is known as the relative age effect.
The findings were published in the Canadian Medical Association Journal.
Lead author Richard Morrow, University of British Columbia, said: "The relative age of children is influencing whether they are diagnosed and treated for ADHD.
"Our study suggests younger, less mature children are inappropriately being labelled and treated. It is important not to expose children to potential harms from unnecessary diagnosis and use of medications."
The side effects of ADHD drugs can include sleep disruption, increased risk of heart problems and slower growth rates. As well, younger children who have been labelled ADHD may be treated differently by teachers and parents, which could lead to negative self-perception and social issues, the authors said.
Coauthor and psychiatrist Jane Garland, University of British Columbia and BC Children's Hospital, said: "This study raises interesting questions for clinicians, teachers and parents.
"We need to ask ourselves what needs to change. For example, attention to relative age of children for their grade and more emphasis on behaviour outside the school setting might be needed in the process of assessment."
The study investigated almost one million schoolchildren in British Columbia over ten years who were between the age of six and 12.
Although the prevalence of ADHD diagnosis and treatment is about three times higher in boys than girls, the effect of relative age applied to both.
Girls born in December were 70 per cent more likely to be diagnosed with ADHD than girls born in January.
The researchers wrote in the journal: "The potential harms of overdiagnosis and overprescribing and the lack of an objective test for ADHD strongly suggest caution be taken in assessing children for this disorder and providing treatment."
Experts in Britain said the findings could mean that ADHD was not not being picked up in the older children in the year group as well as or instead of too many diagnoses being made in the younger children.
Dr Dave Coghill, Reader in Child and Adolescent Psychiatry, at University of Dundee, said: "In a culture where ADHD is over diagnosed it may be that relative age increases the likelihood that younger children in a year will be over diagnosed.
"In cultures where there is under recognition of ADHD such as the UK and Canada the opposite may occur with there being a more appropriate recognition in younger children with under recognition of ADHD in the older children in a year.
"The researchers suggest that this relative age effect is likely to be driven by the teacher. In the UK parents are always intimately involved in the assessment for ADHD and in almost all cases it is them that provide the key information that drives the assessment process. As a consequence it is possible that the UK will be less prone to these relative age effects."
Dr Liz Didcock, mental health lead at the Royal College of Paediatrics and Child Health, said: "This paper adds an interesting perspective on the diagnosis and treatment of ADHD in children.
"It reminds us of the importance of understanding behaviour in its context, particularly in relation to the influence of parental and teacher expectations.
"The potential 'over-medicalisation' of what can be considered 'normal' childhood behaviour is an obvious concern. It is vital that all clinicians from Paediatric and Child and Adolescent Mental Health Services teams are skilled in the assessment of children's development, including their social and emotional health.