Upvote:5
How to Be a Victorian: A Dawn-to-Dusk Guide to Victorian Life by Ruth Goodman discusses this.
The newborn would wear a binder, a simple strap, until about nine months, where it would be replaced -- for both sexes -- by a stay band, looking much like a corset, but stiffened by vertical channels with stout cord. When the child hit seven or eight, girls moved into corsets and the boys left them off.