Disclaimer: I'm not Jo, the world of Harry Potter belongs to J.K Rowling.
16 years old.
Walburga Black turned her steely eyes towards her eldest son who sat huddled in the corner, knees drawn up to his chest, arms wound around himself tightly in an effort to look as small as possible. Scared grey eyes looked up at her through a mass of wavy black hair. At times like these, Walburga wished her husband would simply let her slap some discipline and order into her son's thick head. Alas, but no, Orion Black felt that corporal punishment wasn't suited to the Most Ancient and Noble House of Black. The Blacks were too refined to resort to such pedestrian means of discipline, and as a result Walburga was limited to yelling out threats at the offending 7-year-old.
Walburga took a deep breath, looked at her House Elf Kreacher standing by the door, missing her younger son hiding behind smug elf, and turned back to her eldest. "Werewolves Sirius?" she screeched, "You had the audacity to enter the library, look through the books hidden in the forbidden shelves only because you wanted to prove to little Regulus that Werewolves aren't monsters? The nerve. What were you thinking? I'm just glad Kreacher fetched me when he did."
Regulus, who heard his name being called out straightened, he wasn't little, he wanted to tell his Mother just as much, but knew that right now it would only cause more trouble. Regulus turned to look at his brother instead.
Sirius looked up at his Mother, sat straighter and raised his chin stubbornly. "They are not monsters." His chin wobbled slightly as his Mother narrowed her eyes but he steeled himself further. "Uncle Alphard told me that they only turn so once a month, they're human otherwise. Reggie wouldn't listen to me, I tried to tell him that it was wrong of you to get that awful werewolf skin rug for the parlour. I had to make him see."
Sirius wanted to tell his Mother how Regulus thought that Werewolves were monsters because the rug in the parlour wouldn't turn to human skin when the moon wasn't full, and therefore the beasts couldn't be human, but decided against it. He didn't want his brother to face his Mother's wrath.
Walburga Black took a minute to process the sheer insolence of the child sitting before her. Oh how she wished she could just magic him into obedience. Blasted Orion and his thrice blasted ideals. "I should set you lose around a Werewolf on the next full moon. Let that monster bite you and turn you into one of them. Then we'll see how much you sympathise with them" she spat. Behind her, Regulus' eyes widened in fear.
But Sirius stood, eyes narrowed, body taut. For a moment Walburga thought that he was going to pounce on her. But her son looked at her with an expression she had never seen on a child. "Then I'll simply bite you when I transform. We'll see how you still think of them as monsters."
Walburga was outraged.
Regulus, shocked.
Kreacher looked at Sirius with an expression so far reserved only for Muggles, Half-Bloods and Blood Traitors.
Sirius took that opportunity to sprint pass them all.
Years later, Walburga Black would reflect on that incident and wonder however did she hope for her eldest son to be a Slytherin. That behaviour was Gryffindor impertinence at it's very worst.
Years later, Padfoot would meet Moony; and on a particularly quiet full moon night, Sirius Black, lazing in his newly perfected Animagus form in a dilapidated little house, on the edge of Hogsmeade, would look at the lightly snoring Werewolf, restlessly prancing Stag and a hungry Rat chewing on the laces of a discarded shoe and feel a wave of pure affection and respect for each of them. Though a touch more admiration for the Werewolf. Padfoot would snort, thinking back to his argument with his Mother all those years ago, take this, you old harpy, Werewolves are not monsters, they're just humans who, once a month need a little bit of support and love from their family and friends to get through a tough night.
At 16-years-old, Sirius Black realised that love sometimes meant having to grow fur and learning to walk on four legs instead of two.