The attainment gap between summer-born babies and their older peers gradually narrows during their early education but remains significant even at the end of primary school, according to detailed new analysis.

The disadvantage of being the youngest in an academic year at primary school has been well documented and remains a serious cause for concern for parents, many of whom choose to delay their child’s school start date.

This new research tracks the academic progress of summer-born babies in detail based on standardised termly tests within schools and shows a gradual closing of the gap with older peers as they progress through primary, but it never disappears.

According to this latest research, in reception year, when the gap is at its widest, four- and five-year-olds born in the summer achieve an average of 7.5 percentage points less in maths tests, compared with children who were not born in the summer.

By year 3, at the ages of seven and eight, the gap has narrowed to 5.5 percentage points; the following year it is 4.6 and then by year 6, the final year of primary school, it stands at 3.6 percentage points.

The findings by SchoolDash, an education data analytics company, are based on 1.5m individual pupil assessments in maths and reading, gathered by teachers between 2015 and 2018 in 2,000 state primary schools in England. All results have been anonymised.

Among the other intriguing findings is evidence that at the very start of primary school girls outperform boys in maths, with a “slightly higher” average performance than their male peers. By years 2 or 3, the boys overtake them and remain ahead until the end of primary school.

The analysis also explores gender differences in maths according to topic – boys excel in measures and number work, including counting, place value, rounding and negative numbers. Girls do relatively well in geometry and operations, which includes calculations such as addition, subtraction and multiplication.

Girls are also on average stronger than boys in reading at the start of primary school and they remain ahead until the age of 11.

The data looks at differences between types of school and finds that those with higher proportions of disadvantaged pupils show lower average performance in both maths and reading during the early years of primary education. The gap grows over time.

The primary school data on which the analysis is based has been aggregated from primary school results compiled by teachers who use a programme called RS Assessment, supplied by Hodder Education to 6,000 primary schools in England. SchoolDash looked at a subset of 2,000 schools of those schools.

There is already a great deal of Department for Education data about pupil progress at primary, based on national testing at seven and 11. What is different about this data, according to the analysts, is that it offers a more nuanced picture, showing progress at many different stages of primary education as teachers monitor their students and upload their results to keep close track of their progress.

Timo Hannay, the founder of SchoolDash, added: “Such large-scale datasets of this quality are very rare, especially in primary education. This analysis of millions of results in order to characterise and quantify important trends will help to inform future educational priorities and policies.”