This is a great question, thanks for asking! First, let’s clarify what we mean by a girl. Did you know that a person’s sex and a person’s gender are two different things?!? When someone refers to a person’s sex they are referring to the sex the person was assigned at birth, meaning female (girl), male (boy), or intersex. Gender, is much more personal, it’s how you choose to identify. That means the person gender doesn’t have to be the sex they were assigned at birth. Gender refers to a person’s internal feelings of being a women (girl), man (boy), both, neither, or something else.
Most (but not all) people sex assigned female at birth will be born with a reproductive organ called a uterus. The inner lining of the uterus is what sheds every month and causes a period. As such, most (but not all) people assigned female at birth will experience a period, regardless of their gender. Usually when a person assigned female at birth, reaches the age of 8 to 13 years old, they begin puberty and experience menstruation aka their period.
While most girls will get a period because they have a uterus not all will. Similarly while most boys won’t get a period because they lack uterus and ovaries, some will. It is also important to mention that getting a period doesn’t define you as a women. There a lot of different reasons why a girl (who was assigned female at birth) and identifies as a girl, may not get a period. A lack of period could be caused by a difference in your anatomy that you were born with, medical condition, injury, or medications.
Long story short, regardless of how a person identifies, everybody goes through changes!