(In addition to all the other people i'm not, i am certainly not J. Rowling! Nor do i have a ghost of a claim to her characters or universe. I am but a poor harmless trespasser, trying to shelter in my favorite fantasies from the cutting winds of the mundane world!)
Snape had to admit the curse had been ably and subtly administered. Weariness on Friday afternoon -- well, wasn't he always weary after another week of attempting to teach dunderheads the exact art of potion-making? A few aches on Saturday morning -- nothing a dose of his own soothing syrup couldn't mend. Not until he looked into the mirror in the instructor's break room (returning to class without assuring oneself that one's robes were in good order was never wise) and saw the sudden silver threads in his hair, did he recognize the Senescens Volens.
The wards defending Hogwarts were too well-woven for the curse to have reached him from afar. No, he had his fellow employees to thank for this one. Or possibly the students. Most of the fifth-year Ravenclaws had the skill for it. Quite a few of his own Slytherins had probably seen it used before they ever arrived at school. Senescens Volens had been popular among a certain clique, providing a miserable and frequently untraceable death.
Motive didn't diminish the pool of suspects. His fellow Death-Eater conspirators might judge him (incorrectly) too enviably high in Voldemort's favor, or (correctly) too great a risk to their goals. His fellow conspirators of the Order of the Phoenix might judge him (incorrectly) too great a risk to their goals, or (correctly) too enviably high in Dumbledore's favor. (Snape wondered every time he saw the Headmaster how he justified trust in so suspect a character as himself.) The culprit could be one of the instructors he'd slighted or insulted, one of the students he'd slighted or insulted, one of the House Elves he'd slighted or insulted . . . .
(House Elves were not as universally reliable as the Wizarding World assumed. Lucius Malfoy could testify to it.)
Unfortunately for his unknown enemy, the little known Juvendus Delinquus potion was the ideal antidote to Senescens Volens. He'd be busy well into Sunday brewing it, though.
So much for more pleasant plans, supervising that ass Potter's latest detention. The potent odor of Gadarene muskmelon survived even desiccation. A few hours spent chopping the dried rind, and the boy would find his closest friends keeping their distance. However that gang of troublemakers was certain to draw detention again. The rind would keep.
As for the curse, when Dumbledore returned from his Ministry visit on Monday, Severus would apprise him of the situation. They would discover then what information might be gleaned from it.
A little before midnight on Sunday, measuring yew berries with clawed fingers and a shaking hand, Severus admitted (to himself) that he had made a mistake. The Senescens was proceeding far more swiftly than any variant of the spell he'd ever encountered. It could be a wholly new version of the old favorite, adjusted to render useless old countermeasures. In other words, the potion might be ineffective.
Dumbledore was due back at four in the morning. An ungodly hour. If the potion failed, Severus would die of old age by about 2:30. By 10:30am, the hour of his earliest class, it was possible that someone would notice his absence.
Adder skin, chopped acorns, pine-needles . . . had he added the blades of new grass yet? His bleared eyes couldn't tell. An essential ingredient, and he had no time to brew a second dose. Severus dropped them in.
Honey, wine and roses being among the ingredients, the potion was more palatable than some he'd had to swallow. His hand shook as he measured the dose, but that was infirmity and not agitation. His enemy had erred, if they thought he feared death.
The manner of that death, yes. He had seen enough of the Death Eaters' work for that. The acceleration of the Senescens robbed it of much of its terror though. In a few hours he would be dead, or he would be whole. He could not bring himself to decide which was the more desirable outcome.
He looked about the dark acrid stillroom, the jewel-toned bottles of potions gleaming in the candlelight, the retort and alembic and dozens of customized cauldrons. This was the only place he'd been happy. He raised his beaker in ironic salute, and drank.