Tony was three when he realized the world he lived in was supposed to provide some feedback.
It might have been a bit early, but he was a genius, and he'd been getting the feeling for a while that something wasn't right. Because his mother would have cramps and his father would have headaches (hangovers, Tony corrected himself as he grew older) and even Jarvis had sore throats but Tony had none of that.
Tony was four when it was confirmed. Because he didn't understand why everyone looked so concerned when his leg was at a funny angle. Sure, it made it harder to walk on, but he didn't really care, so why was his mother crying, and Jarvis talking to him with soothing words, reassuring him that everything would be alright.
That was when his parents found out, when he didn't so much as blink when they yanked the bones in his leg back into place, only watching in fascination.
The doctors spoke to his parents outside of the room, where he sat with his leg propped up in plaster, a marker gripped tightly in his pudgy hand, wondering whether he should write the periodic table next, or Newton's laws. His lip reading skills weren't good enough to know what they were saying, but his mother burst into tears, and his father looked angry.
("Stark men are made of iron," he'd told Tony before, and Tony thought it meant they were unbreakable, untouchable, indifferent to everything.)
Tony was five when he found out what it was called. He devoured literature on the subject, but there was so little of it, and even less that was accessible to a five year old.
He was more careful after that, more careful with himself, even if Jarvis had been gentler with him ever since he broke his leg, checking him over during bath time to make sure nothing was out of place.
Because Tony was only five, but he still recognized that, in the end, the only one who would take care of him would be him.
Tony was thirteen when he tried to cure himself. Genetics were fascinating and everything, but once he figured out the gene that the mutation was on, he couldn't do much more than that. Human trials were messy things, and considering he barely hit puberty, he didn't think that was going to work with anyone who would have funded the research.
So he considered it an advantage, and when people asked, told them as much, no matter how untrue it was. (He didn't tell people about when he woke up with a mouth full of blood from biting his tongue during his sleep, or when he didn't realize his wrist was broken until the bruising became dark and prominent, and his range of motion was decreased. He didn't tell them about when he was younger and hadn't figured out a bathroom schedule yet, and had wet himself more than once because he couldn't feel that he had to go. He didn't tell people about having to use a thermometer before stepping in the shower, because otherwise he could die. No one wanted to hear those things, and Tony sure as hell didn't want to talk about them.)
So he let people poke him with pins, and remained completely straight faced throughout the entire thing. Sometimes he got fingers too close to welding torches, brushed by hot pans, rested on warm stoves. He ended up with a lot of burns that he didn't realize he'd received.
It became a nightly thing, checking over his body to make sure he hadn't irreparably damaged it again that day.
And maybe as he grew older he became more reckless, drinking to the point where his entire body meant absolutely nothing to him, more than once ending up in the ER because he lost too much blood from places he didn't even know were bleeding.
But Rhodey always took care of him, even if Tony had come to accept it at a young age that the only one who would care for him was himself.
But it was nice to pretend, at least for a while.