Earlier quoted context omitted.
I don't know if the US is the same as other countries.. but in the UK your birthdate defines how much schooling you get. I'm a summer child so my first year of school consisted of a two week introduction and then the summer holiday. My best friend at school and all of the higher performing kids (except me) were born in the first half of the year so had an extra 6 months of schooling.
I thought everyone in the UK started school together, in September?
It may be different nowadays; it would make way more sense to have everyone start in September. I live in The Netherlands and here kids start as soon as they turn 4. They then spend two-three years in the young kids class depending on maturity.