Some of you seem confused, so here's an abridgement:
Fred and George Weasley, using their ingenious skills at mischief making, had formed their very own de-aging potion, and gave it to the professors. The teachers all turned to the age of 16, or thereabouts.
Thus, the elder teachers (such as Albus and Minerva …and basically everybody else) had no idea who the others were, or even what they were doing there. Severus Snape, being the youngest, knew most of the teachers for who they were, and figured out that something wasn't quite right. He mistook Harry Potter for his father James, and went down to the Gryffindor table.
Sirius Black and Remus Lupin come in.
We learn that Sev is actually a Gryffindor prefect. After some excitement with Grown Up Snape's pensieve (or is it penseive?), we learn that Snape's parents were killed because of their association with the Order of the Phoenix.
The remaining professors, however, decided to have a gigantic party.
Sev and Siri, being twin brothers (something we found out in the pensieve) worked together to get the recipe for Fred and George's potion. It turned out they made up a rather famous potion called the Adulescentia potion.
Moldy Voldie, however, seems to be kidnapping muggles and using the Imperio curse on them, to form an army. He had sent Lucius Malfoy to Hogwarts to find out what happened to Dumbledore, since he seems to be out of the picture (with a hangover, actually. A sixteen year old Dumbledore wouldn't have any idea about Voldemort, and thus wouldn't continue holding up the wards that protect the school. Thus the apparation boundary seems to have dissipated.)
And, of course, as an answer to Severitus's challenge, Harry found out that Snape is his real father. Sev and Lily were married until she was killed.
The Fifth Marauder
By Alarun
Part 9
"My Lord." Lucius Malfoy fell to his knees in front of Voldemort, nearly touching his forehead to the ground in front of him.
"Rise," the serpentine voice hissed impatiently. "What do you have to tell me of Dumbledore's stronghold?"
"I have some very interesting information, My Lord …"
* * *
Sirius Black cackled and rubbed his hands together with maniacal glee. "So, what do you think we're going to do now?" He brought up while he and the resident potions geek walked swiftly toward the dungeons; Snape's stalking was somewhat limited because he was sans long black cloak.
Snape hesitated, further taking away from his grown-up self's unconscious elegance. "There aren't any dangerous ingredients in the students' potion cabinets, are there? Of course there wouldn't be …I wouldn't …there's no way for me to get past my own wards, to my own stores ---I don't know the password."
"Bugger. So much for an antidote, eh?"
Snape kicked a mahogany armor stand, cracking an apologetic smile when it turned toward him. The knight took this as proficient apology and turned back to its proper position, proudly holding up a lance.
"It will wear off, eventually," Snape said finally. "I can't remember the exact dates …It's something like this; you gain a year every week for two weeks, after that you gain a year every half-week for two weeks, and eventually you'll end up gaining a year a day, until you hit your normal age. At least, I hope I remember it correctly. It may be that you gain a year a day until you either die, or find an antidote. Or, if you don't drink an antidote before three weeks …or was it two? …the cure would have no effect and you will continue to age until you die. I need Rare Potions and Poisons; I know it's in there. And I bet the book is in my room. Behind the wards."
"But you know an antidote, right?"
"I think I do. It's complicated, though. If it isn't exactly right ---" Severus suddenly felt lightheaded, and he caught himself on the mahogany stand when his knees buckled. He saw Sirius heavily sit down on the ground, holding his head in between his knees. Must be phantom pains. When twins were especially close, they experienced each other's emotions in a slightly lessened state.
All of the muscles in his left forearm convulsed spasmodically, and the inside of his left forearm felt like it was on fire. Snape clenched the hurt arm against his side, as if pressure would alleviate the pain. He heard Sirius cry out next to him, and he ground his molars together to prevent from screaming.
When the wave of pain eased slightly, Severus and Sirius both stood.
"I have to go. I'm being s-summoned, right?" Snape flinched at his stutter.
"Yes. B-but you can't apparate, you don't know how! And you can't do it here, because of Dumbledore's anti-apparation wards …"
"I'll figure it out. Take this back to the dungeons, and see if you can find the Adulescentia potion in any of the text books." He handed Sirius the paper of ingredients for the youth potion. "It's worth a look, anyway."
Sirius could only nod as he watched his brother run off into the opposite direction, both of them holding their left forearms.
* * *
Severus ran toward the nearest tower and flung open the latched window, jumping onto the wide ledge and watching the wind currents. This side of the castle was facing the lake, and the wind was stronger on this side. If he wasn't careful, he could get his angle wrong and hit the side of Hogwarts.
"Just wing it," He murmured to himself, echoing an earlier memory of Lily.
He stepped down and took two steadying breaths, closing his eyes and willing the pain down. The next time he opened his eyes, the world was magnified greatly, and in an odd color mixing of greys and reds.
Snape spread his animagus gyrfalcon wings, white spotted with grey, and tested them. His left wing was sore, and upon closer inspection he saw the Dark Mark on downy black feathers on the underside.
Cursing, which came out as a predatory birdcall, he hopped up onto the windowsill again. Severus jumped out of the window, opening his wings at an angle to soar toward the ground and pull up at the last possible moment. He inwardly smiled to himself, in spite of his destination, at being the fastest animal in the world.