In the early 1900s, it was common for little boys to wear dresses until they were 6 years old

Up to the early 20th century, little boys were "unbreeched" and wore dresses until about 6 years old. It was a fairly large milestone in a boys life when they were finally "breeched" and started to dress more like what boys dress like today. Historians, and presumably people living during that time, relied on subtle differences between boys and girls at that time to be able to tell them apart.