Responses
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.
don't the secondary gender characteristics develop later in life? around elementary school age?