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According to Portuguese law, students who have completed 6 years of age on 15 September must be enrolled in the first year of primary school in that same school year. But students born between 16 September and 31 December may register, or not, depending on their parents' wishes and the school availability in that school year. This system is designed so that, for example, a student born on 31 December 2014 can register for the next school year (2020/2021), but a student born on 1 January 2015 can only register for the next school year (2021/2022). Although these students have a difference of only one day of age, they will enter the school at very different ages.

The fact that the enrolment date depends on the students' birthday creates what economists and education economists call "cut-off points", i.e. specific moments associated with rules that change the subjects' conditions, in this case the distribution of students between classes and schools. Here, the two cut-off points are 16 September, since the parents of pupils born after this date may, or may not, enrol them in that school year, and clearly 1 January.

A very recent study for Portugal, summarised here using data from more than 660,000 pupils followed between the 1st and 9th grade and another set of 630,000 pupils followed between the 9th and 12th grade, measured the impacts of school entry age in the short and long term. Considering the set of pupils present in the data, the concept of cut-off points can be clearly observed in the figure.

It is clear that students born on the first days of January and those born on the last days of December of the same year and entering school in the same school year have a significant age difference – almost one year! This may be reflected in a difference in maturity and consequently in their school results. It is precisely this cut-off point that makes it possible to find the impact of age on school success, assuming that among students whose birthdays are in the first and last days of the year, nothing distinguishes them significantly except the day of the year in which they were born.

In order to measure the average impact of this age gap on 1st year entry, this study used its results in national exams and tests at the end of the 4th, 6th, 9th, and 12th grades. It concluded that the difference of one year of age at entry to school in the 1st year has positive impacts at the end of the 4th year in Portuguese Language and Mathematics.

It is curious that this effect has an important dimension, similar to the maximum positive impact found in the famous Star programme which reduced class sizes from 22 to 15 students [1]. The impacts are slightly greater for girls, but quite homogeneous among pupils with different socio-economic conditions. The results remain significant for the 2nd cycle and then fade during the 3rd cycle and secondary education. However, at the end of 9th grade, older pupils still have a 2 percentage points lower probability of being retained and a higher probability, also by 2 percentage points, of taking a scientific-humanistic course in secondary education.

However, there is one issue which deserves closer attention. Are these positive results derived from being older at the beginning of the first cycle the result of greater maturity in the school curriculum, or are they simply the result of students being older when taking the exams that serve as a measure of achievement?

In an attempt to answer this question, let us look at another study, this time with Norwegian data, which follows all pupils born between 1962 and 1988 in a total of 600,000 observations. In order to separate the effect of later entry into school from the effect of the age at which the test is taken, these data have information about the IQ test taken by Norwegians at the age of 18 when they registered in the army. In addition, depending on the month of birth, these tests are conducted at different times, and the age of the individual is collected at the time of these tests. In this way it is possible to identify separately the impact of the age of entry to school and the age at which the test is carried out, concluding that the latter effect explains the students' best results. In other words, it is the greater maturity of pupils at the time of taking the tests which serve to gauge knowledge, rather than an impact on their overall school performance, which explains the higher results of pupils born at the beginning of the year compared to pupils born at the end of the year. The same study focuses on long-term impacts such as the higher level of education achieved, wages in the labour market, levels of mental health or claims for social assistance in adulthood. For all these variables, there was little or no impact of a later entry into school.

This result about the long-term impacts of delaying entry to school is not consensual in the literature, as other studies have found positive impacts on wages or the likelihood of reaching management positions in a company. These long-term analyses do not, however, internalise other costs associated with delaying school entry for younger pupils, such as increased costs for pre-school education or later entry into the labour market. There is certainly much to study in relation to this problem.

The results thus point to short-term positive impacts in delaying the entry of the youngest pupils into school, with less consensual results in long-term gains. Positive short-term impacts seem to be justified by different degrees of maturity at the time of assessment.