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During their second year, Izuku accidentally slips when they’re watching a movie (it was Big Hero 6 and he loves that movie!). Before he knows it, he is sitting with his legs curled up and the hand he was using to pick at his lower lip just slips the thumb into his mouth.
No one really notices at first, all content on the movie or sleeping. Izuku just watches, wide-eyed with excitement at the display of heros on the screen.
He’s sitting in between Shouto and Ochako, with Iida on Ochako’s other side and Denki and Sero in front of them on the floor. Kacchan sits on the loveseat by himself to the right of the room and Mina, Kiri, and Tsuyu have the other couch on the other side.
No one notices the thumb-sucking for a good third of the movie.
Izuku isn’t super little or anything, maybe 4 or 5 years old… he just likes putting things in his mouth and his fingers are usually the first to go in. It is a comfort habit of his. It is why his favorite candies are lollipops and suckers; some people would call it an oral fixation.
Really no one notices until Shouto glances over and in his usual calm tone asks, “are you sucking on your thumb, Midoryia?”
He looks over, turn his head and keeping his thumb securely on his tongue, and cocks his head to the side. No one has ever asked him about it; the only person he’s really been around is Kacchan when he is like this and the blond has never asked him about it.
When he glances around, he sees a lot more eyes on him than he was used to. It makes him shrink his head into his neck a little and he doesn’t know what to do. His thumb slips from his lips.
“Is’t bad?” he asks in a voice pitched a little higher than usual.
“What?” Ochako asks right next to him, genuine curiosity in his voice; not that his little mind can comprehend that.
Izuku looks around wildly until he finds a familiar face. The moment he looks over, Kacchan glances away from the movie for the first time to tune in to what is happening.
“Oh shit,” he mutters, sitting up from his reclined spot on the couch.
The curse makes Izuku giggle, yucky feelings disappearing now that he know Kacchan’s here! “Hehe, dats'a bad word,” he giggles.
Katsuki stands up and doesn’t look away from Izuku’s face when he asks, “how old are you right now, Izuku?”
Izuku stops and thinks about it for a second. Then he holds up four fingers.
And that is how the majority of class 2-A finds out about Izuku being an age regressor.