Education in the United Kingdom is devolved, which means four governments make separate decisions about the same years of childhood. The result is a set of arrangements that differ more than most people in any one of them realise.
This is worth knowing for two reasons. If you move, almost everything changes. And the variation is informative: features that seem inevitable in one nation turn out to be conventions when another does without them.
England
Covered at length in the reception year in England, so briefly here. The early years foundation stage runs from birth to the end of reception, organised into prime and specific areas of learning. Almost all children start reception in the September following their fourth birthday, although compulsory school age is the term after the fifth birthday.
England assesses more than the others: a progress check between two and three, a reception baseline assessment, the end of reception profile, and the phonics screening check in Year 1.
Scotland
Scotland's Curriculum for Excellence organises learning into levels rather than year groups, and the early level deliberately spans the pre school years and the first year of primary. The stated intention is continuity across the transition into school, which is the discontinuity England struggles with.
Children start Primary 1 in August, with a cut off that means the age at entry varies by birthday. Scotland has also moved to allow parents of children who would otherwise start before their fifth birthday to defer entry with funded early learning for the additional year, an arrangement that is more generous and more predictable than the discretionary system in England.
Guidance and curriculum materials are published by Education Scotland.
Wales
Wales has undertaken the most thorough rebuild of the four. The Curriculum for Wales replaced the previous arrangements and is organised around areas of learning and experience and a continuum of progression, with schools designing their own curriculum within a national framework.
The Welsh approach also gives statutory status to a set of cross cutting elements and places substantial emphasis on the Welsh language. Inspection is carried out by Estyn, whose framework differs from Ofsted's in structure and in how findings are reported.
The reform is recent enough that its effects are not yet established, and anyone claiming to know how it has worked is going beyond the available evidence.
Northern Ireland
Northern Ireland has the earliest start in the United Kingdom, with children beginning Year 1 at four. The first two years of primary form a Foundation Stage with a curriculum developed by the Council for the Curriculum, Examinations and Assessment, and a strong emphasis on play based learning within those years.
The combination is worth noticing: the earliest entry age in the United Kingdom coupled with an explicitly play based first two years. It is a useful counterexample to the assumption that early entry necessarily means early formality.
Northern Ireland also retains academic selection at the end of primary through tests administered outside the statutory system, which shapes the later primary years in ways the other nations do not experience.
What the comparison is good for
Not ranking. Cross national comparison of educational outcomes is confounded by population differences, funding, what the assessments measure and how they are administered. Attributing a difference in outcomes to a structural feature is not something these comparisons can support, for reasons set out in correlation and cause.
What they are good for is separating the necessary from the conventional. Assessment at four is not necessary, because three of the four do not do it in the same way. A hard break between early years and the national curriculum at five is not necessary, because Scotland's early level spans it. Early entry does not require formal instruction, because Northern Ireland pairs the earliest entry with a play based foundation stage.
Each of those is a design decision that could be otherwise, and knowing that changes how you read a debate that is usually conducted as though the current arrangement were the only possible one.
If you move
Practically, families relocating between the nations encounter more disruption than they expect. Year group placement is not automatic, since the age at which a child enters a given year differs. Curriculum content differs in sequence. Assessment points differ. And the language of reports differs enough to make comparison hard.
The school and the local authority or education authority are the people to ask, and they are used to the question.
The reason to know this
Because it makes the arrangements visible as arrangements. Someone deciding that four year olds should be assessed in their first weeks of school, or that a curriculum should be organised by subject from five, has made a choice with reasons behind it.
Those reasons deserve examination rather than assumption, and the easiest way to start examining them is to look across a border and find people doing it differently and getting on perfectly well. For what the phrase readiness conceals in all this, see what school readiness means.