Researchers in Croatia explored the relationship between the age that pupils begin school and school achievement. They found only a weak relationship in the lower grades of primary school, and at the end of primary schooling the effects are no longer evident.
The study looked at the achievement of fourth- (ages 10 and 11) and eighth-grade pupils (ages 14 and 15) in 844 primary schools in Croatia. Pupils were divided into groups of younger and older school entrants based on the difference between their year of birth and the year of school entry.
In the fourth grade, older entrants performed slightly better in all subjects than those who were younger when they entered school, but these differences in achievement were very small (effect sizes ranged from 0.02 to 0.07). By the eighth grade, there was no difference in achievement between younger and older entrants in the majority of subjects. However, contrary to the fourth grade sample, in the subjects where differences in achievement were found, the younger school entrants outperformed the older school entrants, but the effect sizes were again very small (effect sizes ranged from 0 to 0.12). In both samples, school entrance age explains less than one per cent of the variance in school achievement in different subjects.
Source: The relation between school entrance age and school achievement during primary schooling: Evidence from Croatian primary schools (2012), British Journal of Educational Psychology , 83(4)