How old is Mary Morston in The Sign of Four?

In Sir Arthur Conan Doyle's second novel, The Sign of Four, published in 1890, we are introduced to Miss Mary Morstan.

Miss Morstan entered the room with a firm step and an outward composure of manner. She was a blonde young lady, small, dainty, well gloved, and dressed in the most perfect taste. There was, however, a plainness and simplicity about her costume which bore with it a suggestion of limited means. The dress was a sombre grayish beige, untrimmed and unbraided, and she wore a small turban of the same dull hue, relieved only by a suspicion of white feather in the side. Her face had neither regularity of feature nor beauty of complexion, but her expression was sweet and amiable, and her large blue eyes were singularly spiritual and sympathetic. In an experience of women which extends over many nations and three separate continents, I have never looked upon a face which gave a clearer promise of a refined and sensitive nature.

As to her age, we need to deduce that, as did Doctor Watson at the end of the chapter, from the facts she presented to Sherlock Holmes.

"I was placed, however, in a comfortable boarding establishment at Edinburgh, and there I remained until I was seventeen years of age. In the year 1878 my father, who was senior captain of his regiment, obtained twelve months' leave and came home. He telegraphed to me from London that he had arrived all safe, and directed me to come down at once . . ."

So, seventeen years old in 1878.

"He disappeared upon the 3d of December, 1878,—nearly ten years ago."

Seventeen years old in 1878, nearly ten years ago. Now 26 or 27 years old, depending on the date she went to see Sherlock Holmes and her actual birth date.
Doctor Watson deduced similarly.

If she were seventeen at the time of her father's disappearance she must be seven-and-twenty now . . .

Dr. John Watson and Miss Mary Morstan are married in 1889. Mary is mentioned or appears briefly in The Adventure of the Crooked Man, The Man With the Twisted Lip, and The Boscombe Valley Mystery. Mary died from undisclosed causes at some time after 1891 and before 1894, between The Final Problem and The Adventure of the Empty House.