The Early Years Foundation Stage Profile (EYFSP) is a statutory, summative teacher assessment against nationally prescribed criteria. It is completed for all children in state education in England in summer of the reception year, when September-born children are nearly 6 and summer-born children are turning 5. This research uses quantitative descriptive and regression analyses of de-identified National Pupil Database records spanning 2008–2018 (N = 6 million+). It considers how children’s own age and the mean age of their school year-group peers interplay in determining attribution of the EYFSP ‘Good Level of Development’ (‘GLD’). Birth month is starkly predictive of ‘GLD’ attribution: on average, across years, August-borns are 30 percentage points less likely to be deemed ‘Good’ than September-borns. Moreover, an older year-group predicts lowered chances of ‘GLD’ attribution for children born across all seasons. In 2018, for example, a summer-born child in a much older year-group had an estimated 58% chance of being attributed a ‘GLD’; a comparable summer-born child in a young year-group had an estimated 65% chance; an autumn-born child in an old year-group had an estimated 79% chance; and an autumn-born child in a young year-group had an estimated 83% chance. Results build upon previous research indicating that teachers’ judgements are relative, and also complement studies describing ‘moderation’ requirements forcing teachers to shape EYFSP scores into prescribed patterns within schools. The latest EYFSP revision continues to ignore the dominance of age in determining ‘GLD’ attribution and the effects of context and relative judgement. Therefore the EYFSP cannot be an entirely ‘reliable, valid and accurate’ measure of ‘child development’ as intended by the Department for Education.