The youngest children in their school year may be at a higher risk of mental health problems, research suggests.
Scientists analysed the GP records of more than a million children from across the UK. They found the students who were the youngest in their year group were 30 per cent more likely to be diagnosed with depression by the time they turned 16.
The youngsters were also 36 per cent more at risk of attention deficit hyperactivity disorder (ADHD) and 30 per cent more likely to be diagnosed with an 'intellectual disability'.
The scientists noted children in the same school year as each other can be almost 12 months apart in age.
Younger children may find it harder to concentrate, grasp what they are being taught or maintain friendships with their older peers, they added.
The research was carried out by the London School of Hygiene & Tropical Medicine.
'We've known for a while children who are young in their school year are more likely to have hyperactivity disorders and tend to do less well academically than older children,' lead author Jeremy Brown said.
'However, we believe this is the first evidence of an association between younger relative age in the school year and increases in the diagnosis of depression.'
The scientists analysed the Clinical Practice Research Datalink, an electronic primary care database for more than 700 GP clinics.
Results - published in the journal JAMA Pediatrics - revealed the youngest quartile were around 30 per cent more likely to develop depression than those in the oldest quartile. This remained true after adjusting for factors like socioeconomic status and gender. No difference in risk was found between boy and girls.
To put the results into context, around 800,000 children start primary school in the UK every year. Of these, the scientists predict about 500 more of the youngest in the year might be diagnosed with depression compared with the oldest (2,200 versus 1,700), over the course of their schooling up to 16 years old.
The scientists also found ADHD was more common in children under 10. Their study did not set out to uncover why young children may be more at risk of these conditions. They note, however, 'relative immaturity' compared to their older peers may suggest hyperactivity.
Academic performance and depression have also been linked. And 'relative youth' may lead to 'poorer peer relationships', which could trigger depression. The scientists stress the total number of children diagnosed with depression remains low.
However, they hope their study will encourage more research into the causes of depression in schoolchildren and how it can be prevented.
