high school age

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.

We find that a one-year delay in school entry is associated with a 0.303 decrease in standard deviation of test scores.
This paper examines the effect of school starting age on the academic achievement of junior high school students using the newly available data from the China Education Panel Survey. Regression discontinuity design estimation based on an exogenous entrance cutoff date indicates that a one-year delay is associated with a 0.303 decrease in standard deviations of cognitive scores. However, this negative effect is caused by human capital accumulation prior to primary school entry.

References:

http://www.ego4u.com/en/read-on/countries/usa/school
http://www.sciencedirect.com/science/article/pii/S1043951X17300445
http://www.amalorpavamschool.org/