A basic dictionary will define the word ‘girl’ (obviously in the noun form) as a female child, who is not yet an adult. That’s true. Therefore a girl can be said to range between the ages 0-25 years.
One thing predominant of every girl irrespective of age is the presence of the vagina (every female has one). And as a girl grows, she develops some other parts of her body (If you see any of the following signs, you are still a girl), hair appears in the pubic (vagina) and axillary (armpit) regions, breast starts to get big, her hips expands, she starts to get monthly discharge of blood called menstruation. A girl (you are probably one) will start to see these signs when she is around 11 years of age to around 14 years of age. Whenever these signs come up, don’t be alarmed, you are only reaching a stage called puberty. You should be happy you are growing.
Whenever a girl starts to attain this stage, it is pertinent for the girl not to forget that she is still a girl and she still remains a female child that has not attained adulthood.
In that case, she is not expected to do what adult female (women) do in the case of sexuality. Although the puberty stage gives power to hormones in her body, to push her to behave like adult females, she is still not allowed to behave like adult females. Big breasts, large hips, monthly discharge etc. are not license to becoming sexually active.
Sexual activities is for adult females (and in fact only for married adult females) who have left the stage of being called a girl and have achieved better and greater things like completing their education.