The month in which children begin school can depend on their birthday
Summer-born children do significantly worse in exams than those born earlier in the school year, research suggests.
Analysis of exam results in England's state schools showed August-born pupils were less likely to get five good GCSEs than those born in September.
The biggest factor affecting children's performance was the age at which they sat the tests, the Institute for Fiscal Studies report suggested.
It called for more flexibility in when national tests were taken.
The IFS report compared the percentages of children born in August who achieved the level expected for their year group with those of children born in September.
It found that while 60.7% of September-born girls and 50.3% of September-born boys achieved five GCSEs grade A* to C.
In contrast, 55.2% of August-born girls and 44.2% of August-born boys did so.
The gap in attainment between pupils born in September and those born in August at the end of Key Stage 2 - the last year of primary school - is 14 percentage points.
The report also found evidence that teachers and parents were mistaking poor performance as a result of age for special educational needs.
August-born girls were 72% more likely than September-born girls to be recorded as having special educational needs.
The researchers said: "Our work suggests that these differences predominantly arise because August-born children are almost a year younger when they sit the tests.
"These disparities remain significant at ages 16 and 18, so that date of birth may be influencing decisions over whether they stay in education or leave school and enter the labour market."
The report also suggested that policies linking the month of a child's birth with when they were admitted to school disadvantaged them in the early years.
Testing when ready
"In general, August-born children are slightly better off if they start school in the September of the academic year in which they turn five rather than the January or April as happens in some local authorities," the report said.
The research also found that August-born girls, who received two terms less schooling, faced the additional penalty of 3.8 percentage points in their Key Stage 1 score and 2.5 points in their Key Stage 2 scores.
The researchers suggested that national test results be moderated by age or that greater flexibility be introduced into when the tests were sat.
Flexibility over the age at which children could start school might also reduce the August birth penalty, they added.
Schools Minister Andrew Adonis said the findings of the research would be considered alongside other issues in consultations on the government's 10-year plan to improve the lives of children.
He added: "We are piloting changes to assessments in 500 schools so that children take national tests as soon as they are ready, rather than only at the end of a long Key Stage, as well as more than 30 pilot projects offering free childcare to two-year-olds.
"This is a practical initiative which may also address the birth date issue."
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