Disclaimer: I own nothing.
When they grew up, things changed. Mycroft set up to become the British Government, though Mum assured him many times that it wasn't a single man, so that if Sherlock ever slipped and got discovered it would be to Mycroft himself that they'd bring the issue. And Mycroft would smile and assure that he already employed his brother for the good of the nation, so nobody needed to fret about it. Sherlock would be protected like this. Safe.
But becoming the government was hard work, and Mycroft had no time at all for his teen little brother, just vague prospects of working together in the future. Sherlock, though, didn't need a boss; he needed an older brother right then. The only reasonable deduction he can make from Mycroft's distance is that his brother only meant to exploit him; that this is why he was taught deductions, even – to be used later on. The resentment grew and festered.
Sherlock started hanging out with the worse crowd he could find in retaliation, and soon he realized something. The junkies were destined to be his best friends. He didn't need to hide his wings with them. They would chalk it out to a weird trip.
And Sherlock's wings, by then, had grown to be eagle-like, big, much less feathery and more jet black, shiny quills. They suffered being constricted inside his clothes all the time, no matter how much tailored they were. His wings wanted to stretch, and so Sherlock did, with his friends now, knowing Mycroft would have a conniption if he found out (well, he wouldn't be the only one to be honest) and loving it all the more for it.
Still, at the start it was an occasional thing. Sherlock still had hopes, no matter how half-formed. But years went past, the hate of his peers for his 'goddamned party trick' and more simply him never relented, and Sherlock grew tired. Caught between hate and despise on one side, Mycroft's pressuring and his parents' still loving, but ultimately uncomprehending attitude at home (how could they understand? They weren't freaks of nature), and the utter dullness of the rest of the world, he upgraded his drugs of choice.
It was an escape, but a trap at the same time. He told himself that he wasn't addicted, that a thing as pedestrian as addiction was beneath him. He could stop anytime. He just didn't wish to, because what was there to be lucid for?
Once he reasoned that if he became normal, he wouldn't be hated anymore. Somehow, this for him translated not into finally getting social cues, but into getting rid of his wings. He nicked a scalpel and a large quantity of morphine from an hospital (he wasn't about to undergo an operation without it; he wasn't stupid), he found a empty, mostly clean room in one of his usual haunts and then...HIs memories grew hazy after that point.
He woke up later, in hospital, and feeling like absolute shit. And if that wasn't bad enough, Mycroft – who was never there when Sherlock wanted him – was at his bedside. Grumbling about "what were you thinking?" and "how much stupid can you get?" and "you overdosed, Sherlock. You'd be dead if I didn't save you."
"Well, don't save me next time," Sherlock replied bitingly. Or tried to. In reality he mumbled something indistinct, but he was still bone tired.
That episode resulted in a scar on his left shoulder, that nobody would ever see, and Mycroft thinking he could still do whatever he pleased with Sherlock's life attempting to check him in rehab. Multiple times. Sherlock only developed a talent for escapism that would be very useful if he ever found himself in a tight spot.
Why couldn't the git see that Sherlock had nothing to get clean for? Yes, his parents loved him. It wasn't enough to build a life on, and that's what he was supposed to be doing at his age. Find his way. Know what to do with himself.
He found the solution to that problem while searching for more drugs. He stumbled on a crime scene near one of his haunts and solved it for the idiots buzzing around, if only to obtain that they'd leave the premises so his usual dealer could come back. Naturally, he got arrested, because they didn't believe him when he said that he wasn't a witness, and insisted that if so he must have been involved in the crime then. A good deed never went unpunished.
Luckily Lestrade gave him a chance to prove the authenticity of his deductive powers. He presented Sherlock with a number of past, solved crime scene photos and Sherlock guessed (sorry; deduced) correctly each time.
When Lestrade tried to release him (and ordered to arrest the man this lanky genius told them was guilty), Sherlock realized that he hadn't itched for drug as long as he'd been there and half pleaded, half demanded, "Get me more cases."
The inspector indulged him, so he solved seven cold cases before leaving the station, grinning for a verbal accord with Lestrade to let him help in more cases still, "As long as you get clean. And stay that way." The inspector recognized addiction when he saw it, but it didn't make him dismiss Sherlock. It was strange.
The young man had just found a way to keep his brain from rotting and be useful. Stop more people from being potentially hurt by helping the police arrest murderers and...anything interesting that'd come their way. Be part of something good, for once.
It was worth a few sacrifices, surely? It was worth getting clean. It would only mean exchanging one form of stimulation for another. It was worth only giving a breather to his wings inside his home and living a life of permanent hiding, too. Just as his family had always suggested.
Sherlock called his brother. "I'm ready," he said softly. Mycroft would understand.