Ok so when I discovered that Hinata is actually older than Kageyama I came up with a headcanon
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The Karasuno team is sitting in a circle and introducing themselves with "Hi, my name is (first name) (last name), I'm (age), my birthday is (birthday) and I like to (something other than volleyball) (Daichi forced them to say something else, mainly because of Hinata lol)." (or something along those lines) and it's Kageyama's turn. He says "Kageyama Tobio, 16, December 22nd, milk." And there's this lil gasp and Kageyama turns to glare at whoever did it and sees Hinata with his eyes shining about to totally lose it and Kageyama's face is just like 'what is it you motherfucker' and Hinata is laughing really hard, holding his sides and rolling on the floor gasping for breath and manages to wheeze out "I'm older than you!" before collapsing into another fit of giggles and the entire Karasuno team (minus Nishinoya and Tanaka cuz you KNOW they're besties and they already knew Hinata's birthday) is just staring in shock because they all assumed that Hinata would be the youngest out of them all. And Kageyama turns bright red, gets up, and storms out of the room. And the entire team is left to either laugh their asses off (Tanaka and Noya) be concerned that Hinata isn't getting enough oxygen (Sugawara) or stare at the doorway in shock (everyone else).
