I'm not a teacher, I'm a mom. I think the maturity level is the most important
thing. My two sons were very bright, but very immature. My older son was
on the cusp, could have gone to school with the older group, but we decided against it, and it was a good thing. (we could have kept him out of school
until he was 10 and it would have been a good thing) but i digress.
If she's mature, doesn't mind being the littlest, and the teacher is cool with it; it's a great idea. That was my personal experience, (being the youngest
and smallest in my own school experience) and i don't think it hurt me a bit (it was good for my ego, instead!)

mimi