uk school system ages

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Children born between 1 April and 31 August can start the September after they turn 5.
Your child must start full-time education once they reach compulsory school age. This is on 31 December, 31 March or 31 August following their fifth birthday – whichever comes first. If your child’s fifth birthday is on one of those dates then they reach compulsory school age on that date.

References:

http://www.gov.uk/schools-admissions/school-starting-age
http://www.gov.uk/schools-admissions/school-starting-age