There was a thread on this a month or so back. Everyone agreed that it should be up to the flower girl to decide if she'd like the honor or not... she may feel like she's too old, but on the other hand she may feel left out if you dont ask her.
How about calling her a Junior bridesmaid? Picking a dress that is not as cutesy as a flowergirl but not as sophisticated as a bridesmaid? This age is big into "pre-teen" so this might work.
I had a flower girl age 5 and a junior bridesmaid age 9. The flower girl had on a store bought party dress and the jr bridesmaid wore the same as my bridesmaids. It really is up to you and the little girl. Ask her opinion.