When he was three, the world was fascinating. There were so many things he wanted to learn, so many questions to be asked. His family could teach him, by giving him answers and showing him how things worked. It was completely natural to ask and be answered.
When he was four, the world made less sense the more he learned. He devoured book after book, needing to know why. Why his father had left without a word. Why Mummy wouldn't tell him when Dad would be back. There had to be answers somewhere.
When he was seven, the world was vast and dizzying. His brother and his books were the only ones who could answer his questions. The other children, with their dull games and derisive laughter, didn't want to know about the world like he did. They didn't understand him, and he frightened them, though he didn't mean to.
When he was twelve, the world had turned against him. He had begun to realize that he was different, and he wondered what was wrong with him. He hated the rest of the world for misunderstanding him. His anger flared, white-hot and lethal as a bomb blast, at the smallest provocation.
When he was fourteen, the world had become incredibly complex. He knew things about people just by looking that everyone else had to be told. There was information everywhere and it threw itself at him from all sides. The violin settled his mind and helped him think. He kept his room spotless because he needed to be able to control something, anything, while the world spun around him in a whirlwind.
When he was seventeen, the world was boring. It moved in predictable patterns that had ceased to interest him. The fury that had once exploded outwards had pivoted and now festered inside him, a capricious pet turned on its master. It made him cold and bitter, and he sought artificial, chemical ways to feel.
When he was twenty, the world was exactly the same but entirely different. His roommates understood him less than the children on the playground had, but now their laughter was hidden behind oily words and false smiles. Information still poured in from all sides, but he categorized it and saved what mattered. His room was an enormous mess.
When he was twenty-four, the world consisted of dirty streets, cold nights and desperate bids for money. His flat, which he rarely visited, was kept up by his brother. He spent his days on the streets, pickpocketing and shoplifting to stay alive long enough to get his next fix. He avoided his brother's omnipresent surveillance and slept on rooftops where he couldn't be found. His violin gathered dust and slowly lost its tuning in the unused flat.
When he was twenty-seven, the world had regained some of its interest. Lestrade let him solve the interesting crimes, and his brother left him alone more often. He visited his mother and played his violin again. Bart's allowed him use of the laboratory. He was content, and only half as restless as he had been before.
When he was thirty-three, the world was fascinating. There were questions he still wanted to find the answers to, and his mind was as sharp as it had ever been. His partner, the ex-army doctor, presented the most interesting puzzle he had ever attempted to solve. For once, he was happy.