A key stage is a stage of the state education system in England, Northern Ireland, the Isle of Man and the British Overseas Territory of Gibraltar setting the educational knowledge expected of students at various ages.
The Key Stages were first defined in the Education Reform Act 1988 to accompany the first introduction of the national curriculum.
The precise definition of each of the main 4 key stages is age-related, incorporating all pupils of a particular age at the beginning of each academic year.
The key stages were designed to fit with the most prevalent structures which had already grown up in the education system over the previous 100 years of development.
Again, described by Sir William Henry Hadow, this took pupils up to the standardised break at age 11.