Boys and girls began to wear corsets from toddlerhood to promote good posture. These corsets would not be waisted at all - they were supports for the back. We think of the word "corset" today as indicating something very shapely and sexual, but for about a century it simply meant an undergarment for the upper body that was stiffened to some degree. Once a boy was put into trousers, his coat would pull his shoulders back, but girls remained in corsets that gradually developed a figure.

This page from an 1869 magazine or pattern catalogue shows a progression of corsets for young people. There's a corset for a child (that is, gender-neutral) from 1 to 2 years, for a girl of 8 to 10, and for a girl of 12 to 14.