All individuals are different and I have no objection to recognizing that fact. What I do object to is the presumption that all girls are the same as each other (and that all boys are the same as each other) thus denying their individuality.
At any given age, not all girls will be at the same stage of intellectual and social development nor will every girl be more or less advanced than every boy. Discriminating solely by sex ignores all the relevant factors. Punishing girls more harshly (or is it more leniently? That’s never clear.) because they are presumed to be more (or less?) advanced than boys of the same age is clearly unfair.