Hi sunflower, I have boys rather than girls, my eldest is 7 and a half and is a bright boy, but I am sure that at his age (if he was a girl!) he would have no comprehension of why his body was changing no matter how much i explained it to him. At eight, he won't be even walking to school on his own never mind dealing with the ability to procreate!
A colleague was saying that her daughter who will be 9 in February was complaining of cramps and she wondered if that was the start of puberty and we all said no way, but looks like she could be right.