Christmas Postcards used to be quite the fad. As were those for birthdays, anniversaries, and other holidays.
Sometime in my grandmother's early life, she received a postcard to wish her a Merry Christmas.
Christmas cards continued to be in postcard format until, in 1915, someone found there wasn't enough room to write a long enough note on a postcard, and the book format was designed (ironic that the Christmas card started to eliminate writing letters, and then expanded to make room for writing a letter!).
Since the postcard format was going out of style, I think we can assume that this card to Gladys likely occurred in the first 10 years of her life (before 1924). I assume when she was even younger based on the dates of the other postcards.