School grade by age
School year groups are set by age on a cut-off date, not by age today. England uses 1 September, so a child must be five by then to start Reception. Most US states use dates between 1 August and 1 October, varying by state.
How to work out a school year
Where a birthday falls relative to the cut-off matters more than most parents expect. Within a single year group the oldest and youngest children can be almost twelve months apart, and the relative age effect: younger children in a cohort performing measurably worse on average. Is well documented and persists into secondary school. It is one of the better-established findings in education research and one of the least acted upon.
Questions
1 September. A child must be five by then to start Reception that academic year.
It varies by state, typically between 1 August and 1 October. Check locally.
In England, born between April and August: the youngest in their year group, with a documented disadvantage.
In England parents may request deferral for summer-born children, though it is decided case by case.
The relative age effect is well documented and persists for years, though it narrows with age.