Disclaimer: Fullmetal Alchemist is © Arakawa Hiromu, Viz, Bones, Funimation, Square Enix, etc. This is a nonprofit fanwork.
What most people didn't realize about Roy Mustang was that he was a genius.
At a very early age he had taken to scrawling on his bedroom walls and floor with crayon, much to his mother's despair. He didn't speak and he was sullen with strangers, preferring to exist in a world that orbited around a very certain center - himself.
She was convinced that he was slow for his age as were all his teachers. When the other children were busy learning their letters or numbers, young master Mustang was drawing fantastic pictures on his books, his desk, any blank surface he could get his hands on – even the back of a classmate's white dress. Which made it all the stranger that he showed zero interest in finger painting the duckies or kitties that his peers were busily occupied with during art class.
His mother was just about ready to take him out of school and schedule sessions with a specialist the school headmaster had referred to her when the incident occurred.
His father came home from a two and a half year long military campaign.
He listened to his wife's concerns about their son with half an ear and vaguely assured her that they would do something about it. Secretly, he promised himself that he would do nothing of the sort. It would be an absolute disgrace for a Mustang to see a so-called specialist because of "mental deficiencies." He had his family pride on the line and he was certain it was only a stage that his son would soon grow out of.
That notion firmly established in his mind, he decided to treat his son like any normal child until he behaved like any other normal child. A few evenings later he noticed his son looking intently at a thick leather bound volume a fellow officer had lent him.
Mustang Sr. fancied himself a bit of an academic and had a particular interest in alchemy, though sadly no talent with which to support that interest, and had a sizeable portion of his personal library dedicated to that area of study. Mindful of his son's proficiency with a crayon and the cost of that particular book, he walked over to him and clapped a hand on Roy's dark hair, pulling the book out of his reach.
"Now, now," he said, laughing amiably, all the while attempting to see if Roy would understand him, "before you can read such things you have to learn your letters properly in school like all the other children."
Most children's first words are, "Mama" or "Dada" or something of a similarly heart-warming nature. Roy Mustang's first words were, said with some irritation, "But I was just beginning to understand the subtle radiance variations in the Metereonic array versus the Skylsdell model."
Roy did indeed get taken out of school, and, equipped with a private tutor, blazed his way through a series of advanced upper division classes aimed at prepping someone for university. He might indeed have gone to university had he not finally acquired adequate understanding of alchemy.
After all, it had only taken him that long to work out all the basics because his mother kept having his schematics painted over and his teacher kept confiscating his notes.
His statement to his father one evening that he wished to become an alchemist working within the military suitably impressed his father to such a degree that his tutor was suspended and Roy was given leave to pursue his own studies and interests. For four years he lived and breathed alchemy, and his mother dared not so much as touch his walls.
He preferred to conduct all his experiments in his room. It had become a great deal less flammable than the rest of the house.
He became a nationally certified alchemist at the age of twelve. However, there wasn't much anyone trusted a twelve year old to do so he was relegated to menial tasks and hours upon hours of boredom. He occupied himself with self-study, but even that was only enough to take the edge off his boredom. It was then Roy Mustang discovered girls.
They ruined a perfectly good academic career.