According to a 1922 New York Times Magazine article, the average age of female protagonists in love stories had increased by about a decade since around 1900 or so.

In the appreciation of the public of today, mature beauty eclipses mere girlish loveliness. It was not always so. Fifteen or twenty years ago the heroines of love stories were 18 or 20 years old, arguing a contemporary public view that this was the most interesting time of life in woman. A canvass of the recent novels shows that the average heroine today is 28 years old.

Curiously, this came during a period where the exact opposite trend was occurring in “real life.” The median age of first marriage went down between 1900 and 1920, for both women and men alike:

The Age of Heroines

Published: Sunday, October 1, 1922