How old is Vera in the short story "The Open Window"?

Saki specifies Vera's age in the opening sentence of the story.

"My aunt will be down presently, Mr. Nuttel," said a very self-possessed young lady of fifteen; "in the meantime you must try and put up with me."

The author needed to have someone set Framton Nuttel up for the shock he was going to receive when he saw the three hunters approaching the open window. Vera is appropriate for this purpose for several reasons....

Saki specifies Vera's age in the opening sentence of the story.

"My aunt will be down presently, Mr. Nuttel," said a very self-possessed young lady of fifteen; "in the meantime you must try and put up with me."

The author needed to have someone set Framton Nuttel up for the shock he was going to receive when he saw the three hunters approaching the open window. Vera is appropriate for this purpose for several reasons. She is just old enough to be credible, yet just young enough to have the mischievous spirit common among many girls. Being a Victorian girl, Vera has no freedom. She is stuck in this big house with nothing to do. She must spend much of her time reading. And because she is so bored, she probably reads escapist literature by writers like H. Ryder Haggard and Rudyard Kipling, which is where she could get ideas about men being sucked into bogs and being chased by pariah dogs in India. She has a good imagination, but she could not have imagined those graveyard dogs without some input. Girls her age are also very good at making faces, and we can imagine her faked look of goggle-eyed horror when she sees the hunters approaching.

Vera probably resents being used as a substitute hostess. She suspects she is being trained for the kind of life that is in store for her when she gets married and turns into a sort of basket case like her Aunt Sappleton, who has been brainwashed by listening to nothing but male talk about killing birds for many years. Vera is rebellious and takes it out on poor Framton Nuttel. Instead of playing the "good hostess" as expected, she becomes the hostess from hell.

Vera is motivated by boredom. She has heard the same things so many times that she knows precisely what her aunt is going to say when she gets down and knows that one of the hunters will start singing, "I said, Bertie, why do you bound?" Vera wants some excitement in her life, and she has to provide it herself.