I suffered similar growing up a tomboy, and as an adult looking back on it I can chalk it up as a societal pressure thing to have one's kids meet a certain standard in behavior and appearance. There's a sense of embarrassment and "What will the neighbors think??" coming off the parents as they're trying to make their kids look and act a certain way so they can bound their chest and boast "Look! I parented good!"
So, yeah, unfortunately the tumblrite has a semblance of a point there. It is a bit of a societal pressure placed upon girls in particular to look and act a certain way once the hormones start pouring in. I didn't grow up with brothers or male cousins, so I have no frame of reference, but I would assume this pressure to 'grow up' comes a bit later for men because there's no obvious signs that their baby-faced little boy is becoming a man until like... 14 when they get their very first chin hair.
Biologically, women start to mature faster.
Socially, woman are pressured to mature faster because it shows faster.
Kinda sucks, but what can you do but go through the motions?