Young Girl In Shorts

In the 1890s, knee pants (an early type of short pants) became the standard wear for American boys. Many urban school portraits from the 1890s show all but the oldest boys wearing knee pants.
American boys normally wore knee pants with short stockings. This began to change after the 1900s when American boys began wearing knickerbockers, while short pants became more popular in Europe.
In the 30's, shorts started to be worn for casual comfort (e.g. outdoor and athletic activities) by both men and women. However, it was still taboo to wear shorts outside certain activities.
Since about World War II, when soldiers often served in tropical locations, adult men have worn shorts more often, especially in summer weather, but the perception of shorts as being only for young boys took several decades to change and to some extent still exists in certain circles. By the late 20th century however it became common for men to wear shorts as casual wear in summer.