The validity of ADHD diagnosis in Australia is again being questioned with a WA study showing the youngest children are more likely to be receiving medication.

The youngest children in class are more likely than their older classmates to receive medication for ADHD (Attention Deficit Hpyeractivity Disorder), a study has found.

The results of the WA study, published in the Medical Journal of Australia, has again raised concerns children are being misdiagnosed with the psychiatric disorder and medicated for what could simply be age-related immaturity.

As a new school year looms, the study has also added to the debate about the best age for a child to start kindergarten.

"Allowing parents to decide when their child is ready for school, could prevent misdiagnosis," lead researcher Dr Martin Whitely.

"Alternatively, the greater age range within a class that occurs when there is increased flexibility may increase the late birthdates effect. Unless we do the research we simply won't know the answer to this important question," Dr Whitely said.

Researchers at Curtin University compared the proportion of WA children in the early and late months of a recommended school year intake who received at least one prescription for an ADHD medication in 2013.

Among the children aged 6-10, those born in June - the last month of a recommended school year-year intake - were about twice as likely to have received ADHD medication than those born the previous July.

For children aged 11-15 years, the effect was less marked "but still significant".

"Similar differences were found when comparing children born in the first three (or six) months and the last three (or six) months of the school year intake," the authors wrote.

Delayed school entry is much less common in WA than other states but the results are consistent with four large scale Northern Hemisphere studies, the study authors said.

"Similar findings in North America indicated that developmental immaturity is mislabelled as a mental disorder and unnecessarily treated with stimulant medication," the authors wrote.

A child with ADHD normally has three main problems - inattention, impulsivity and overactivity.

Dr Nick Kowalenko from the Royal Australian and New Zealand College of Psychiatrists says there isn't enough detail in the research but there is cause for concern.

Deciding to prescribe kids as young as six with antipsychotic drugs is always taken with great care and after thorough discussions with the parents, he said.

"Because the guidelines are actually pretty tight most practitioners actually use them. There must be some maverick practitioners and rogue prescribers but they would be very few."

Diagnosing ADHD can sometimes be complicated because it can mimic post traumatic stress disorder or anxiety and can also co-occur with other conditions like autism.

That said, the diagnosis criteria is very well refined, Dr Kowalenko said.

"The question is does everybody follow them accurately?"