She's at the age where she wants to act like a girl
She's reached the age where she wants to do girly things
She's at the age where she wants to be treated like a girl
「女の子"をしたい」＝ be girly / do girly things / act like a girl
「年頃」＝ the age
「女の子"をしたい年頃」＝ She's at the age where she wants to act like a girl / She's reached the age where she wants to do girly things / She's at the age where she wants to be treated like a girl
イギリス 
