From third to sixth grade, boys tend to their identification with "masculine" personality traits, whereas girls' identification with "feminine" personality traits .

strengthen; declines.    

Explanation:

A masculine personality trait is related to men whereas feminine personality trait is related to a female. If a specific woman is displaying assertive or dominant characteristics then she would be considered as bossy however if a man does the same he will be represented as a boss and therefore he is taking control on something.

As a boy grows up, his masculinity increases because he explores the world and started taking responsibility for various things as per the societal norms and therefore girl femininity declines with age as she is being involved in being independent.