From the Smithsonian: Social convention of 1884, when FDR was photographed at age 2 1/2, dictated that boys wore dresses until age 6 or 7, also the time of their first haircut.

a June 1918 article from the trade publication Earnshaw's Infants' Department said, “The generally accepted rule is pink for the boys, and blue for the girls. The reason is that pink, being a more decided and stronger color, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.”

In 1927, Time magazine printed a chart showing sex-appropriate colors for girls and boys according to leading U.S. stores. In Boston, Filene’s told parents to dress boys in pink. So did Best & Co. in New York City, Halle’s in Cleveland and Marshall Field in Chicago.

Today’s color dictate wasn’t established until the 1940s, as a result of Americans’ preferences as interpreted by manufacturers and retailers.