they do change a little bit. it's more pronounced for girls who sing, like in the shower or in a choir. most of us know that boys' voices crack and change when they hit puberty. girls' voices change a bit too. like I said, it's more pronounced for singers.
Well, some do and some don't, my gir- well, ex-girlfriend did not, but my gest friends got slightly higher, my cuzens got lower. It all deppends on your genes, ask your parents what happended with them.