Found by Lee Bear - Childhood is the Kingdom Where Nobody Dies by MemoryDragon
http://www.archiveofourown.org/works/904690?view_adult=true
Tony is young, i think he may have been de-aged by the baddies and doesn't remember anytging from his life prior to the age he is physically, all of the otyer Avengers are adults
I believe the story starts with him having already been kidnapped
He finds that the person in the room next to his is Steve, they talk through the wall. I think Tony's room is a storage cupboard and steve talks him into turning on the light, once he does he figures out how to rig a broom up so that when the shelves fall it breaks the door and he manages to escape but he can't get steve out
I think the other avengers were being controlled with collars and remotes
In one of the rooms he is hiding from the the kidnappers and the hulk kills a kidnapper and tony keeps thinking about their eyes that stayed open
Somehow eventually all the avengers escape the mind controle and figure out that Tony is 'their' Tony but de-aged
Thats all i can remember for now, any help would be appreciated