high school senior age

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.
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.

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.tas.gov.au/parents-carers/parent-fact-sheets/age-child-startleave-school/
http://www.sa.gov.au/topics/education-and-learning/general-information/sa-education-system