The paper aims to verify two hypotheses. The first states that the differentiation of initial 1st grade pupil skills is negatively correlated with age at entry to school. This was tested according to findings from the ERI study Six and seven year olds at the start of school. The variation of reading and writing scores of 1164 pupils aged 5.9–7.9 years who were beginning 1st grade decreased in subsequent age quartiles. The second hypothesis – that classroom differentiation of pupils’ initial skills in 1st grade is positively correlated with the birthdate effect in 3rd grade – was tested on data of 4838 pupils from 254 classrooms drawn from the 2011 PIRLS and TIMSS studies in Poland. Skills were evaluated on the basis of parental reports. Hierarchical linear analysis (gender, SES and school location controlled) showed that the greater the differentiation of language skills in 1st grade, the greater the birthdate effect in mathematics in 3rd grade. This result suggests that school entry age is of lesser importance than the methods used to reduce differences in children’s school readiness at the onset of education.