
i love verite, but it's like, he used to present as an adult before. and he only changed his appearance because the king asked him to. neither verite not we the readers know the reason why he used to present as an adult male, but it REALLY feels weird seeing this plot development

He is a child, tho. I remember reading that he was using the older form because that was his brother (the one that cursed him and make him lose his memory). He is two hundred years old, but looks like maturity works different for fairies. When he came out of the mirror he has his original child body.

why does it lowkey feel like kotaro is projecting on shinpei because shinpei easily and nonchalantly crossed the boundaries that kotaro constantly lashed himself for years to not cross because he caused trouble for her (noting the burden of society that was on both their heads which shinpei was pretty much exempt from), and not only that, but also won her heart and influenced her enough to sway her decisions, one that kotaro had every chance to do but punished himself for wanting to?
don't the secondary gender characteristics develop later in life? around elementary school age?
It depends on the author. Each rule of omegaverse differs depending on the author
It depends on the world setting. How the author set it up. Chinese ABO and Japan ABO sometimes have diff concept when it comes to manifestation.
But it's really just depends on the author how she wants it to be. In this case their second gender is decided by birth.