Nathaniel Hawthorne, "Dr. Heidegger’s Experiment" (1837) from Nathaniel Hawthorne: Tales and Sketches:
I first read "Dr. Heidegger's Experiment" in ... high school? Middle school? Elementary school?! In any case, I can't remember a time when I thought "bad people get a chance to be young again--this will go well." Maybe it's just baked in to our collective unconscious that wishes go awry--especially wishes of mad scientists, like Hawthorne's Rappaccini and Aylmer ("Rappaccini's Daughter," "The Birth-mark"), to name his most popular two. Not that those are his only two: even The Scarlet Letter has a sinister, learned man, while his notebooks are full of science-y things as the kernels for stories. Another technology Hawthorne is fascinated by: photography, as in the House of the Seven Gables.
The LoA page notes the recurrence of the sinister scientist in Hawthorne and his repeated motif of youth's return--which I would actually widen out a little to something like "the return of history, however ghostly": the parade of ancestors in Seven Gables, the play-acting of history in Blithedale Romance, the sentimental replay of history in his sketches, etc. (But can you tell: I read the heck out of Seven Gables, am not so up-to-date on my other Hawthorniana.)
OK, fine, this story is very much in Hawthorne's motif-wheelhouse. What about the story itself?
Hawthorne doesn't mince words about how old the people are here: the first two sentences say "old, venerable, white-bearded, withered," and "Widow." He also launches in directly to the story of their unhappy and sinful pasts--so we're not exactly on their side. "Venerable" doesn't equal "respectable." And after we hear about that, the next entire page is taken up with how sinister Dr. Heidegger is, with his book of magic and mirror that shows his dead patients. Which also clearly shows that we're in Hawthorne's liminal fantasy/science space.
(My favorite example of this is actually in "The Birth-mark": when sinister Aylmer in the 1700s (I think) shows his wife these "magical" things that, for his 1800s audience, are all nameable technologies. Magical light show--well, that's just a zoetrope. And so on.)
I also have to shout-out the trope of the pre-experiment experiment that Hawthorne uses: before the people take the Fountain of Youth water, Heidegger uses it on a rose that springs back to life. So of course, right before the people get old again, the rose shows the limited power of this rejuvenation. This is a trope that still gets used today, with the scientist noticing--oh no!--that the rat they used before human testing has become sick/reverted to being dumb/etc.
But what really stands out here isn't just that the people grow young, but that they revert immediately to the sins of their past: the old drunken soldier starts singing drinking songs, even though he isn't in a bar or drinking; the old merchant starts mumbling about deals, even though there are no deals to make. This is why I think we shouldn't just classify this as "becoming young again" but as "history repeating." It's a phantasmagoric, fantasy situation.
And ruling over this dream is Dr. Heidegger, who may be mysterious, but occupies a less sinister position than many of Hawthorne's other doctors. He's not trying to ruin anyone's life, or even necessarily putting his own research above other people's health--he's not the one who ruins the rest of the Fountain of Youth water. He may be as stern as Time but he's also as unmalicious as Time. Science is not mad here--just inexorable.