Timing of school enrollment may vary considerably due to children who were enrolled early / delayed or skipped / repeated a school year. Accordingly, the age range within school grades includes older-than-keyage (OTK) and younger-than-keyage (YTK) children. Studies examining academic performance of OTK and YTK children versus keyage children reported large differences in academic performance. However, there are no studies available that examined physical fitness of OTK and YTK versus keyage

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... dren. The aim of this study was to compare physical fitness of OTK (N = 25,086) and YTK (N = 1,885) children versus keyage children (N = 108,295) in a sample of German third graders. Physical fitness tests comprised cardiorespiratory endurance, coordination, speed, lower, and upper limbs muscle power. Predictions of physical fitness performance for YTK and OTK children were estimated using data from keyage children by taking age, sex, school, and cohort into account. The difference between observed and predicted z-scores yielded a delta z-score that was used as a dependent variable in the linear mixed models. Findings indicate that OTK children showed impaired performance compared to keyage children, especially in coordination, and that YTK children outperformed keyage children, especially in coordination. Teachers should be aware that despite their age advantage, OTK children often show impaired fitness compared to keyage children.