Incidentally, if you're wondering how well is this rule adhered to in practice, the dating website has done an extensive analysis of age preferences amongst their members.
It recalls a past which is not yet 'period'." One cause of the change in young women's behavior was World War I which ended in November 1918.
A lot of young men did not return home from the war, which left many women alone in their home.
The slang word "flapper", describing a young woman, is sometimes supposed to refer to a young bird flapping its wings while learning to fly.
However, it may derive from an earlier use in northern England to mean "teenage girl", referring to one whose hair is not yet put up and whose plaited pigtail "flapped" on her back; The word appeared in print as early as 1903 in the United Kingdom and 1904 in the United States, when novelist Desmond Coke used it in his college story of Oxford life, Sandford of Merton: "There's a stunning flapper".
By the mid-1930s in Britain, although still occasionally used, the word "flapper" had become associated with the past.