There is such a stubborn refusal among many (most?) people to believe that even from before birth boys and girls are treated differently, sometimes in quite subtle ways, and that it's those differences in expectations and experiences that lead to differences in later behaviour.
I think it would be super difficult to really raise an agender child, though, although much, much easier to raise a girl with fewer gender norms than it is a boy.
Boys being called Charlotte or wearing dresses and makeup are still definitely Not Ok while girls being called Charlie and having short hair and wearing jeans is fine.