My OLDER brother keeps calling me unc and it's pissing me off

I turned 18m last Wednesday. My older brother is 24. I call him "unc" as a joke. Everyone knows what it means and means, but if you don't know what it means it basically means uncle for short, and as a joke you're calling that person old.

I call my brother "unc" as a joke because he's literally in his 20s. He graduated college, he lives in his own place, all that stuff. I know 24 isn't actually old.. If I were to be genuine then I would say once you're 30 then you're actually an "unc". I did it as a HARMLESS joke.

However, the moment I turned 18 my brother starts to also call me unc because I'm "legally an adult" now, so I'm now in the same tree as him. I don't even feel like an adult. It makes zero sense for him to call me unc when I'm literally YOUNGER than him. Also, he probably just wants me to feel bad and make me feel like I'm old.

He's basically just doing this for revenge and he's just acting like a kid.. I just feel like if he says it again, I would want to punch him in the face. (But I can't cause he'd easily beat me). Can you guys just tell my brother he's being an asshole. I will probably show this to him as well.