They shouldn't worry about it unless the vulva becomes itchy or irritated. If it does, a girl should visit her doctor or clinician. Boys testicles get bigger and hang lower. Usually one hangs lower than the other. Their penises grow larger and get hard more often - and unexpectedly.
Boys gradually start growing beards after their voices change.
Sometimes boys' breasts become tender and somewhat enlarged, too. This is called gynecomastia. It usually lasts only a year or two, and then the size of the breasts returns to normal. We inherit our size, eye color, and skin color from our parents. We also inherit large breasts or small breasts, thin legs or thick legs, big penises or small penises, and thick beards or thin beards.