I've been around the definitions game several times over, going through a few, joining sites and for want of a better expression falling out of them until I finally found something that clicked. What I feel is that cusp between being outside of the baby stage and before the onset of being a teen with the maturation that comes with it (or in some respects at least for not) even if legally I'm an adult with all the stuff that goes with it so I generally call myself an adult little girl (alg) because it takes Little Girl and adds to it my legal adulthood status.
Christine, I so agree your differentiation between "Sissy" and Little Girl as much that in honesty I have to say I at one time fought it because it seems me it's the being a Little Girl that counts so much more than just appearing or dressing like and that there's a big difference between the whole (forced) Emasculation/humiliation/reduced to a girl theme and the love of being that girl, playing, dressing up from within that Little Girl.