what age do you go to secondary school

If you turn 16 between 1 October and the end of February you can leave at the start of the Christmas holidays in that school year.
You can leave school on the last Friday in June, as long as you’ll be 16 by the end of that school year’s summer holidays.

You can find out what year your children need to start school with our school-age calculator below.
In Western Australia, children can start their education in Kindergarten, however compulsory schooling starts the following year in Pre-primary.

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This means that a child who has turned five on or by 1 January in any year must start Prep in that year, or be provided with approved home education.
Every child, from the year after they turn 5 must be enrolled at and attending school, or an approved home education program. Children can also attend Kindergarten from 4 years of age.

References:

http://www.education.wa.edu.au/enrolling-in-school
http://www.goodschoolsguide.co.uk/choosing-a-school/state-schools/secondary-school-admissions
http://www.education.tas.gov.au/parents-carers/parent-fact-sheets/age-child-startleave-school/
http://prosperoteaching.com/from-overseas/uk-key-stages-by-age-year/