As this study says in its introduction, “Myopia is a common and serious problem because of its high prevalence, ocular morbidity, and cost of treatment. Myopia commonly occurs in children during their early school years and increases in magnitude as they increase in age. Only a small number of studies have reported the number of new cases of myopia that develop in children during their primary school years.”

We briefly excerpt a topline summary from the study and link to it on the NIH site below.

Objective – To report the percentage of new cases of myopia in 4,927 children, age 5 – 16 years, who participated in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study between 1989 and 2009.

Methods –  Subjects were children who were not myopic (right eye’s cycloplegic autorefraction of less myopia/more hyperopoia than −0.75 D in both principal meridians) at study entry. A new case was a diagnosis of myopia (right eye’s cycloplegic autorefraction of −0.75 D or more myopia in both principal meridians) after study entry.

Conclusions –  Sixteen percent of children enrolled in the CLEERE Study developed myopia during their school-age years. The percentage increased yearly until age 11 years, after which it decreased. New cases of myopia varied by ethnic/racial group.

Here’s the study on the NIH website.