@Tandora
If gender is just knowing what sex you are, then my kids both know what sex they are - they refer to their parts by the correct names and say things like "I am a boy because I have a willy and you are a girl because you have a vulva". My kids are 3 and 5, btw. They share a bath often, so they know why they have different bits.
Is this enough to be "their sense of their own sex" to have fixed their gender at their young ages?
If they think they are trans when they are 14, what has happened? Does gender suddenly just change depending on what ideas and stereotypes they get exposed to? In which case, why is it more important than sex, which is a constant through everyone's life and a key determinant of strength, aggression, bone density, propensity for violent crime and risk taking, likelihood of getting pregnant?