How old is Vera from "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."

Saki must have started with the idea of having someone tell a completely false story about three men being sucked into a bog three years ago, so that when the three men returned from hunting, the visitor will take them to be ghosts and will flee in terror. The author chose a fifteen-year-old girl to tell the false story to the visitor because she would be just old enough to be believable but just young enough to be so full of mischief that she would invent such a tale. At age fifteen, Vera is also old enough to be playing hostess as a temporary stand-in for her aunt. The fact that Vera is in the position of a hostess lends further credibility to her story.

We can imagine that at age fifteen, Vera is bored with her confinement in this country manor where nobody ever talks about anything but shooting birds on the moor. Maybe she has sometimes wished that the hunters really would get sucked into a bog. Girls like Vera had no freedom in Victorian times. She must spend most of her time reading books, and because of being stuck in such a narrow world, she undoubtedly chooses escapist reading about travel and adventure. This would give her some of her ideas about hunters being sucked into bogs and men being set upon by pariah dogs in India.

Girls like Vera had nothing to look forward to but marriage. She senses that she is being groomed to be a hostess like her aunt and that she will be expected to talk the same kind of drivel. This may explain why Vera does just the opposite of what is expected of her. She is so convincing that poor Framton believes the three men approaching the open window at tea time must be living dead who have finally returned for tea. This belief is reinforced when Vera's aunt cries:

"Here they are at last! Just in time for tea, and don't they look as if they were muddy up to the eyes!"

In saying they look as if they were muddy up to the eyes, Vera's Aunt reinforces the idea that these ghostly figures have been buried in a bog for the past three years and have just climbed out.

Vera is the ideal character to tell the frightening story and set Framton up for the scare of his life. Framton is also the ideal victim for this precocious girl. He is down here in the peaceful English countryside for a "nerve cure" and has been ordered to have "complete rest, an absence of mental excitement, and avoidance of anything in the nature of violent physical exercise." 

"The Open Window" is a good example of how a skillful fiction writer will create characters to fit the needs of his plot. Mrs. Sappleton is also perfect for the part she has to play.