https://schoolsweek.co.uk/late-starting ... cs-scores/
Late-starting summer-born pupils don’t get better phonics scores
Summer-born pupils who start reception after turning five do not significantly outperform those pupils who start aged four.
According to new research from the Department for Education, summer-born pupils granted permission to start school a year later only score on average 0.87 points more on their first formal test than summer-borns who start school aged four.
Some councils allow summer-born pupils – those born between April and August – to start school a year later than they normally would. Parents claim that being in a younger cohort is fairer on summer-born pupils and boosts their performance.
But today’s research found the improvement in phonics score made by those allowed to delay starting school was “not statistically significant”.
Do read the whole piece - it isn't long and includes links.