Children born in August perform significantly worse than classmates who were born earlier in the academic year at all four key stages in Welsh schools, research shows.
Assembly government statistics show a gradual decrease in achievement the later in the school year a child is born: those born in September or October get significantly better results in assessments than those born in July or August.
The most marked difference in results is in key stages 1 and 3. At the age of seven, there is a 14 percentage point difference between the results of pupils born in September and August in the same academic year; at 14 there is an 11 percentage point difference.
The figures provide new evidence of the so-called August birth penalty. Because of the way the academic year falls, a child born on August 31 starts school and sits exams a year earlier than a child born only one day later.
Experts have argued that the system is unfair, but teachers have different views on its impact.
Sue Edgar, headteacher of Acton Park Infants school in Wrexham, said: "It can have an impact on their learning if they are born later in the year. But I wouldn't say that it's of significant concern to us. We tend to look at children at their stages of learning rather than their age."
Colin James, head of Glyncoed Comprehensive in Ebbw Vale, said: "It's not something we have ever noticed, although I'm not surprised.
"Monitoring systems in school are key to pupil progress, and if pupils aren't achieving, for whatever reason, I would be very disappointed if we didn't pick up on it."
The birth penalty was highlighted in a 2007 study of school records in England and Wales by the Institute for Fiscal Studies. It called for "urgent" action to address the issue, including adjusting test scores to take account of age.
Experts say the development of the play-led foundation phase for three to seven-year-olds in Wales will address the problem.
Catriona Williams, chief executive of the charity Children in Wales, said teachers must have a good understanding of child development and take levels of maturity into account when making assessments of their ability.
"Evidence from developmental psychology suggests that children between the ages of four and five may not be ready, developmentally, for formal education and the foundation phase in Wales is an important policy initiative reflecting this fact," she said.
"As birth date effects appear to be greatly reduced in countries where formal education begins at a later age, we would expect this also to be the case in the future in Wales too."
A spokesperson for the Assembly government said: "Our education provision is designed so that all learners, regardless of month of birth or other factors, should have the opportunities they need to reach their full potential," adding that teachers were expected to track individual pupils' progress so that they can better target their support."