Fics like And I was so young when I behaved twenty-five (yet now, I find I've grown into a tall child)

They tell him that he's gotten de-aged. That the world has changed drastically since he was fifteen. That Yoohyun is now some kind of multi-billionaire. "People have superpowers now!" The girl, Yerim, he reminds himself, tells him. "No they don't." Yoojin counters. Yerim immediately proves him wrong by summoning a little ball of water and twisting it into animal shapes. "I've been practicing!" She says proudly. "What do you think?” “Wow!” He whispers, looking at the little water-dog doing tricks in the palm of Yerim’s hand. And for a moment he is just a boy discovering magic is real. "Cool!" "I have powers too hyung!" This strange version of his baby brother says, before summoning an entire ball of fire in his hand. "Oh my god fire! Put it out!" He screams. Or: Yoojin gets de-aged and gives Yoohyun grey hairs as a result

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