Your question refers of course to the description of the reflection of the four youthful guests in the mirror that hung in Dr. Heidegger's study that is referred to later on in the story as the four guests engage in the same youthful frolics and foibles as they had once engaged in long ago in their youth. Consider the following description:
Yet, by a strange deception, owing to the duskiness of the chamber, and the antique dresses which they still wore, the tall mirror is said to have reflected the figures of the three old, gray, withered grandsires, ridiculously contending for the skinny ugliness of a shriveled grandam.
Let us remember that we are told that the mirror is magic at the beginning of the story. The mirror can be therefore said to symbolise the truth behind the illusion of the magic water. However their outer appearance has changed, the mirror seems to reflect that this is just a temporary transformation and that the old age of these characters will swiftly return, if indeed it is not actually just an illusion. Therefore it is perfectly possible to argue that the mirror shows that the guests didn't actually become young at all. My own personal feeling is that they did become young, but the mirror points towards the brief and temporary nature of the magical transformation.