The new short story was published on the writer's Pottermore website and sees Harry reunite with his old friends at a Hogwarts School reunion.
The 1,500-word essay describes the now 34-year-old Harry as having "threads of silver" in his black hair.
Ron Weasley, played in the hit films by Rupert Grint, has aged less well with his ginger hair "thinning slightly".
Ron's wife Hermione, Harry's wife Ginny, and their children all feature in the story.
Rowling also gives fans a hint of Harry's new life by describing a mysterious cut over his cheekbone which signifies his membership of a top-secret group of wizards called the Aurors.
The story is set during a Quidditch tournament - the fictional sport played by the student wizards on flying broomsticks in the best-selling books.
The Harry Potter author, who lives in Edinburgh, has just published her second crime thriller under the pseudonym Robert Galbraith.
The real identity of Robert Galbraith was revealed last year and sent the first novel, The Cuckoo's Calling, to the top of book charts.