what age does high school start

At the age of five or six, the children attend elementary school (also known as grade school or grammar school), which last six years. The fist year at elementary school is called kindergarten.
In the USA, children start school when they are five or six years old. Depending on the state, schooling is compulsory until the age of 16 or 18. Children younger than five can go to a nursery school or preschool.

Most children start school full-time in the September after they turn 4.
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.

• starting Kindergarten – 4 years old by 30 June 2021
• starting Pre-primary, the first year of compulsory school – 5 years old by 30 June 2021
• starting Year 7, the first year of secondary school
• changing schools.

You need to apply to enrol your children in school for 2021 if they are:

References:

http://www.gov.uk/schools-admissions/school-starting-age
http://www.education.wa.edu.au/enrolling-in-school
http://www.ypo.co.uk/product/detail/education-and-learning/curriculum/530993