The 1,500-word story, published on the writer's Pottermore website, 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".
His wife Hermione, Harry's wife Ginny and their children all feature in the story, which is set during a Quidditch tournament.
Rowling also gives fans a hint of Harry's new life by describing a mysterious cut over his cheekbone that signifies his membership of a top-secret group of wizards called the Aurors.
The author has just published her second crime thriller under the pseudonym Robert Galbraith.