That's a pointless argument, but whatever, I'm not above it, so let me add something.
Of course Luffy acts in a childish manner in many aspects, but, at the same time, in the world of One Piece, he isn't really treated like a child, which I think is the argument Shinuki is making here. He is technically of age in One Piece world (at least where he's born), and he is surrounded by characters that are older than him and already working, or living by themselves, whatever. Overall he has a level of freedom and agency in his attitudes, that, combined with how the world treats him (no one really tries to make him not do something because he's too young, for instance), makes it plausible that he is considered a young adult. But then again, his personality is indeed that of a child, I don't think there's denying that. I don't think neither of you are wrong, lol, it's more a matter of perspective.