Estimating the age of a window requires physical examination and possibly historical records or expert analysis.
However, I can provide some general information on historical window sizes. Older windows, especially those in cottages or houses built before the 20th century, were often smaller than modern windows. The use of standard sizing became more common in the 20th century with the rise of mass production and modern construction techniques.
If the original exterior door height was 5 feet 8 inches, it suggests that the window might have been relatively small, consistent with earlier architectural styles. Still, this alone is not enough to accurately determine the window's age.
To estimate the age of the window, you may want to consult a local historian, preservation expert, or architect who specializes in historical buildings. They may be able to provide a more informed assessment based on architectural features, construction methods, and local building history. Additionally, examining any available historical records or documents related to your cottage could also provide valuable clues.