New research suggests that, while birth date remains a significant factor in performance throughout primary school, the gap narrows significantly when pupils transfer to secondary.
Authors Dougal Hutchison and Caroline Sharp, of the National Foundation for Educational Research, speculate that the change in school allows summer-born children to escape academic expectations - both theirs and their teachers' - established when they were the smallest and youngest in the class.
"It's one of the major messages of hope from our paper," said Mr Hutchison, the foundation's chief statistician. "It's almost as if there is a fresh start when they get to secondary school. There is still an effect, but it's below what is educationally significant.
"When summer-born children start school, they are the smallest and youngest in the class, and it's very easy to assume they are not the brightest. Teachers should always think - is this child really quite bright but just younger than the rest?"
He and Ms Sharp will present their findings at the annual conference of the British Educational Research Association next month.
They suggest that, even when summer-born pupils have the same amount of schooling as their peers, their self-esteem may be affected by being younger and apparently less able.
Age-related difference in the primary years could also be perpetuated by the early age at which UK children start school, setting by ability, and national tests which do not take age into account.
Using the Suffolk Reading Test, the researchers compared the performances of three groups of London children at ages six, eight and 10, and tested them again two years later. Data was collected for more than 7,000 youngsters.
In all three groups, autumn-born children outperformed those born in the summer. The gap closed substantially between the ages of six and eight, but significant differences in performance continued throughout the primary years.
However, another big improvement between the ages of 10 and 12 meant that by secondary school, date of birth was no longer educationally significant.
"When pupils move onto the larger horizons of secondary school with new friends, new subjects and new teachers, expectations may be open to re negotiation," say the researchers.
It could be that secondary schools provide a "fresh start", enabling the younger children in a year group to catch up with their older peers, they conclude.
The report, "A lasting legacy", is available from the NFER library. Tel. 01753 574 123