This study aims to contribute to the debate on the impact of the students´ age differences within a grade on academic achievement. Due to the existence of a cutoff date for school entrance, the age difference between the oldest and the youngest pupils in a grade can be as large as 12 months. The literature offers a large amount of research on this topic with various and inconsistent findings. Stipek (2002) in her comprehensive report presents research supporting the advantage of being older for school outcomes which, however, generally tends to decline through the upper grades (Crone and Whitehurst (1999) found this decline as early as at the end of the first grade); research which found no significant differences between older and younger pupils; and even research in which younger students achieved better test results in reading and math. Better test outcomes of older students report McEwan and Shapiro (2008) or Puhani and Weber (2007). Bedard and Dhuey (2006) found that age difference between the youngest and the oldest children in the grade causes initial maturity differences with long-lasting effects. Elder and Lubotsky (2009) present a contrary view claiming that better early school performance of older children is largely driven by the accumulation of skills prior to school attendance and the difference fades away quickly in first school grades. From this point of view, more important than natural maturation is the quality time spent in kindergarten or in similar facilities.

In the present study we focused on age-related differences in reading literacy achievement in more detail. The data analyzed are from first two cycles of PIRLS – PIRLS 2001 and PIRLS 2006. PIRLS – Progress in International Reading Literacy Study assesses the reading literacy of students in the 4^th grade. Apart from the overall test score, we investigated a few items from two test blocks (The Little Lump of Clay – literal block and Antarctica – informational block) that were used in both study cycles and released after PIRLS 2006. We chose items covering each of four reading comprehension processes assessed in the study - focus on and retrieve explicitly stated information; make straightforward inferences; interpret and integrate ideas and information; examine and evaluate content, language, and textual elements. Two items belong to Low Benchmark, two items belong to Intermediate Benchmark and four items belong to High Benchmark.

We addressed the following research questions: Is the overall PIRLS score of older students higher than of younger students? Do older students achieve higher scores for both purposes of reading assessed in the study – literal and informational? Do older students achieve higher scores for all the reading comprehension processes assessed in the study? What proportion of older and younger students reach low, intermediate and high benchmark for particular test items? Is the pattern of differences found between older and younger student similar in PIRLS 2001 and PIRLS 2006 and what are the trends?

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